diff --git a/lang_tests/inst_var_at_put.som b/lang_tests/inst_var_at_put.som
new file mode 100644
index 00000000..347e1233
--- /dev/null
+++ b/lang_tests/inst_var_at_put.som
@@ -0,0 +1,19 @@
+"
+VM:
+  status: success
+  stdout:
+    5
+    instance of inst_var_at_put
+    6
+"
+
+inst_var_at_put = (
+    | x |
+
+    run = (
+        x := 5.
+        (self instVarAt: 1) println.
+        (self instVarAt: 1 put: 6) println.
+        x println.
+    )
+)
diff --git a/lang_tests/inst_var_at_put_bad_idx.som b/lang_tests/inst_var_at_put_bad_idx.som
new file mode 100644
index 00000000..36fe2477
--- /dev/null
+++ b/lang_tests/inst_var_at_put_bad_idx.som
@@ -0,0 +1,18 @@
+"
+VM:
+  status: error
+  stderr:
+    Traceback...
+    ...
+    Index 2 not valid for array of length 1.
+"
+
+inst_var_at_put_bad_idx = (
+    | x |
+
+    run = (
+        x := 5.
+        self instVarAt: 2 put: 6.
+        x println.
+    )
+)
diff --git a/src/lib/vm/core.rs b/src/lib/vm/core.rs
index 0bafcfd4..e21b5544 100644
--- a/src/lib/vm/core.rs
+++ b/src/lib/vm/core.rs
@@ -822,7 +822,14 @@ impl VM {
                 self.stack.push(v);
                 SendReturn::Val
             }
-            Primitive::InstVarAtPut => unimplemented!(),
+            Primitive::InstVarAtPut => {
+                let v = self.stack.pop();
+                let n = stry!(self.stack.pop().as_usize(self));
+                let inst = stry!(rcv.tobj(self));
+                stry!(inst.inst_var_at_put(self, n, v));
+                self.stack.push(rcv);
+                SendReturn::Val
+            }
             Primitive::InstVarNamed => unimplemented!(),
             Primitive::InvokeOnWith => todo!(),
             Primitive::IsDigits => unimplemented!(),
diff --git a/src/lib/vm/objects/mod.rs b/src/lib/vm/objects/mod.rs
index 66018ff0..14001e87 100644
--- a/src/lib/vm/objects/mod.rs
+++ b/src/lib/vm/objects/mod.rs
@@ -117,6 +117,21 @@ pub trait Obj: std::fmt::Debug {
         }
     }
 
+    /// Return the instance variable at `i` (using SOM indexing).
+    fn inst_var_at_put(&self, vm: &VM, i: usize, v: Val) -> Result<(), Box<VMError>> {
+        if i > 0 && i <= self.num_inst_vars() {
+            Ok(unsafe { self.unchecked_inst_var_set(i - 1, v) })
+        } else {
+            Err(VMError::new(
+                vm,
+                VMErrorKind::IndexError {
+                    tried: i,
+                    max: self.num_inst_vars(),
+                },
+            ))
+        }
+    }
+
     /// Lookup an instance variable in this object. If `usize` exceeds the number of instance
     /// variables this will lead to undefined behaviour.
     unsafe fn unchecked_inst_var_get(&self, _: usize) -> Val {