diff --git a/lib/linguist/heuristics.yml b/lib/linguist/heuristics.yml
index 923303e6d..d5af8a4fd 100644
--- a/lib/linguist/heuristics.yml
+++ b/lib/linguist/heuristics.yml
@@ -749,6 +749,11 @@ disambiguations:
   rules:
   - language: STL
     pattern: '\A\s*solid(?:$|\s)[\s\S]*^endsolid(?:$|\s)'
+- extensions: ['.svx']
+  rules:
+  - language: Survex data
+    pattern: '\A(;|\*[^*]+$)'
+  - language: mdsvex
 - extensions: ['.sw']
   rules:
   - language: Sway