diff --git a/km3io/offline.py b/km3io/offline.py
index d38578d60de688b9de16ec809bee81dcf92ca7ac..1ff221881f2d1077eaa1eea69e4626925c8f19f2 100644
--- a/km3io/offline.py
+++ b/km3io/offline.py
@@ -41,6 +41,7 @@ class OfflineReader(EventReader):
             "dir_y": "hits.dir.y",
             "dir_z": "hits.dir.z",
             "tot": "hits.tot",
+            "a": "mc_hits.a",  # hit amplitude (in p.e.)
             "trig": "hits.trig",  # non-zero if the hit is a triggered hit
         },
         "mc_hits": {