diff --git a/query-algebrizer/src/clauses/mod.rs b/query-algebrizer/src/clauses/mod.rs
index e457987a..7ee93032 100644
--- a/query-algebrizer/src/clauses/mod.rs
+++ b/query-algebrizer/src/clauses/mod.rs
@@ -611,13 +611,8 @@ impl ConjoiningClauses {
     }
 
     /// Ensure that the given place has the correct types to be a tx-id.
-    /// Right now this is mostly unimplemented: we fail hard if anything but a placeholder is
-    /// present.
     fn constrain_to_tx(&mut self, tx: &PatternNonValuePlace) {
-        match *tx {
-            PatternNonValuePlace::Placeholder => (),
-            _ => unimplemented!(),           // TODO: #440.
-        }
+        self.constrain_to_ref(tx);
     }
 
     /// Ensure that the given place can be an entity, and is congruent with existing types.
diff --git a/query-algebrizer/src/clauses/pattern.rs b/query-algebrizer/src/clauses/pattern.rs
index 7874ebdf..516816b6 100644
--- a/query-algebrizer/src/clauses/pattern.rs
+++ b/query-algebrizer/src/clauses/pattern.rs
@@ -81,7 +81,7 @@ impl ConjoiningClauses {
         // Sorry for the duplication; Rust makes it a pain to abstract this.
 
         // The transaction part of a pattern must be an entid, variable, or placeholder.
-        self.constrain_to_tx(&pattern.tx);            // See #440.
+        self.constrain_to_tx(&pattern.tx);         
         self.constrain_to_ref(&pattern.entity);
         self.constrain_to_ref(&pattern.attribute);
 
@@ -238,9 +238,27 @@ impl ConjoiningClauses {
                 if value_type.is_none() {
                     self.wheres.add_intersection(ColumnConstraint::HasType(col.clone(), typed_value_type));
                 }
-
             },
         }
+
+        match pattern.tx {
+            PatternNonValuePlace::Placeholder => (),
+            PatternNonValuePlace::Variable(ref v) => {
+                self.bind_column_to_var(schema, col.clone(), DatomsColumn::Tx, v.clone());
+            },
+            PatternNonValuePlace::Entid(entid) => {
+                self.constrain_column_to_entity(col.clone(), DatomsColumn::Tx, entid);
+            },
+            PatternNonValuePlace::Ident(ref ident) => {
+                if let Some(entid) = self.entid_for_ident(schema, ident.as_ref()) {
+                    self.constrain_column_to_entity(col.clone(), DatomsColumn::Tx, entid)
+                } else {
+                    // A resolution failure means we're done here.
+                    self.mark_known_empty(EmptyBecause::UnresolvedIdent(ident.cloned()));
+                    return;
+                }
+            }
+        }
     }
 
     pub fn apply_pattern<'s, 'p>(&mut self, schema: &'s Schema, pattern: Pattern) {
diff --git a/tests/query.rs b/tests/query.rs
index 9a1f73e9..6bf2853a 100644
--- a/tests/query.rs
+++ b/tests/query.rs
@@ -232,11 +232,9 @@ fn test_instants_and_uuids() {
     conn.transact(&mut c, r#"[
         [:db/add "u" :foo/uuid #uuid "cf62d552-6569-4d1b-b667-04703041dfc4"]
     ]"#).unwrap();
-
-    // We don't yet support getting the tx from a pattern (#440), so run wild.
     let r = conn.q_once(&mut c,
                         r#"[:find [?x ?u ?when]
-                            :where [?x :foo/uuid ?u]
+                            :where [?x :foo/uuid ?u ?tx]
                                    [?tx :db/txInstant ?when]]"#, None);
     match r {
         Result::Ok(QueryResults::Tuple(Some(vals))) => {
@@ -257,6 +255,72 @@ fn test_instants_and_uuids() {
     }
 }
 
+#[test]
+fn test_tx() {
+    let mut c = new_connection("").expect("Couldn't open conn.");
+    let mut conn = Conn::connect(&mut c).expect("Couldn't open DB.");
+    conn.transact(&mut c, r#"[
+        [:db/add "s" :db/ident :foo/uuid]
+        [:db/add "s" :db/valueType :db.type/uuid]
+        [:db/add "s" :db/cardinality :db.cardinality/one]
+    ]"#).expect("successful transaction");
+
+    let t = conn.transact(&mut c, r#"[
+        [:db/add "u" :foo/uuid #uuid "cf62d552-6569-4d1b-b667-04703041dfc4"]
+    ]"#).expect("successful transaction");
+
+    conn.transact(&mut c, r#"[
+        [:db/add "u" :foo/uuid #uuid "550e8400-e29b-41d4-a716-446655440000"]
+    ]"#).expect("successful transaction");
+
+    let r = conn.q_once(&mut c,
+                        r#"[:find ?tx 
+                            :where [?x :foo/uuid #uuid "cf62d552-6569-4d1b-b667-04703041dfc4" ?tx]]"#, None);
+    match r {
+        Result::Ok(QueryResults::Rel(ref v)) => {
+            assert_eq!(*v, vec![
+                vec![TypedValue::Ref(t.tx_id),]
+            ]);
+        },
+        _ => panic!("Expected query to work."),
+    }
+}
+
+#[test]
+fn test_tx_as_input() {
+    let mut c = new_connection("").expect("Couldn't open conn.");
+    let mut conn = Conn::connect(&mut c).expect("Couldn't open DB.");
+    conn.transact(&mut c, r#"[
+        [:db/add "s" :db/ident :foo/uuid]
+        [:db/add "s" :db/valueType :db.type/uuid]
+        [:db/add "s" :db/cardinality :db.cardinality/one]
+    ]"#).expect("successful transaction");
+    conn.transact(&mut c, r#"[
+        [:db/add "u" :foo/uuid #uuid "550e8400-e29b-41d4-a716-446655440000"]
+    ]"#).expect("successful transaction");
+    let t = conn.transact(&mut c, r#"[
+        [:db/add "u" :foo/uuid #uuid "cf62d552-6569-4d1b-b667-04703041dfc4"]
+    ]"#).expect("successful transaction");
+    conn.transact(&mut c, r#"[
+        [:db/add "u" :foo/uuid #uuid "267bab92-ee39-4ca2-b7f0-1163a85af1fb"]
+    ]"#).expect("successful transaction");
+
+    let tx = (Variable::from_valid_name("?tx"), TypedValue::Ref(t.tx_id));
+    let inputs = QueryInputs::with_value_sequence(vec![tx]);
+    let r = conn.q_once(&mut c,
+                        r#"[:find ?uuid 
+                            :in ?tx
+                            :where [?x :foo/uuid ?uuid ?tx]]"#, inputs);
+    match r {
+        Result::Ok(QueryResults::Rel(ref v)) => {
+            assert_eq!(*v, vec![
+                vec![TypedValue::Uuid(Uuid::from_str("cf62d552-6569-4d1b-b667-04703041dfc4").expect("Valid UUID")),]
+            ]);
+        },
+        _ => panic!("Expected query to work."),
+    }
+}
+
 #[test]
 fn test_fulltext() {
     let mut c = new_connection("").expect("Couldn't open conn.");