diff --git a/bwa.1 b/bwa.1
index 994f96a..ee4e4f3 100644
--- a/bwa.1
+++ b/bwa.1
@@ -202,7 +202,7 @@ Discard a MEM if it has more than
 .I INT
 occurence in the genome. This is an insensitive parameter. [500]
 .TP
-.BI -D \ INT
+.BI -D \ FLOAT
 Drop chains shorter than
 .I FLOAT
 fraction of the longest overlapping chain [0.5]