diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,9 @@
+# [0.3.0.0] -- 2025-07-06
+
+[0.3.0.0]: https://github.com/GaloisInc/oughta/releases/tag/v0.3.0.0
+
+- Add `check_not(s: String)`, which asserts that `s` is *not* in `text`.
+
 # [0.2.0.0] -- 2025-05-02
 
 [0.2.0.0]: https://github.com/GaloisInc/oughta/releases/tag/v0.2.0.0
diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -197,6 +197,8 @@
 
 - `check(s: String)`: Find `s` in `text`. Seek to after the end of `s`. Like
   LLVM FileCheck's `CHECK`, or [Expect's][expect] `expect`.
+- `check_not(s: String)`: Assert that `s` is *not* in `text`. Do not seek. Like
+  LLVM FileCheck's `CHECK-NOT`.
 - `checkln(s: String)`: `checkln(s)` is equivalent to `check(s .. "\n")`.
 - `here(s: String)`: Check that `text` beings with `s`. Seek to after the end
   of `s`.
diff --git a/oughta.cabal b/oughta.cabal
--- a/oughta.cabal
+++ b/oughta.cabal
@@ -1,6 +1,6 @@
 cabal-version: 3.0
 name: oughta
-version: 0.2.0.0
+version: 0.3.0.0
 author: Galois, Inc.
 maintainer: grease@galois.com
 copyright: Galois, Inc. 2025
@@ -158,7 +158,7 @@
   hs-source-dirs: test
   main-is: Test.hs
   build-depends:
-    oughta >= 0.2.0.0,
+    oughta >= 0.3.0.0,
   build-depends:
     bytestring,
     directory,
diff --git a/src/Oughta/oughta.lua b/src/Oughta/oughta.lua
--- a/src/Oughta/oughta.lua
+++ b/src/Oughta/oughta.lua
@@ -12,6 +12,19 @@
   end
 end
 
+function check_not(needle)
+  if needle == "" then
+    match(0)
+    return
+  end
+  local start, _ = string.find(text, needle, 1, true)
+  if start == nil then
+    match(0)
+  else
+    fail()
+  end
+end
+
 function checkln(needle)
   check(needle .. "\n")
 end
@@ -28,4 +41,3 @@
 function hereln(needle)
   here(needle .. "\n")
 end
-
diff --git a/test-data/fail/check-not.txt b/test-data/fail/check-not.txt
new file mode 100644
--- /dev/null
+++ b/test-data/fail/check-not.txt
@@ -0,0 +1,3 @@
+# check_not "foo"
+foo
+; check "❌"
diff --git a/test-data/pass/check-not.txt b/test-data/pass/check-not.txt
new file mode 100644
--- /dev/null
+++ b/test-data/pass/check-not.txt
@@ -0,0 +1,5 @@
+# check_not "fourth"
+first
+second
+third
+; match_on_line(1)