diff --git a/pkg/obiformats/fastaseq_read.go b/pkg/obiformats/fastaseq_read.go
index eb6e9cf..c902857 100644
--- a/pkg/obiformats/fastaseq_read.go
+++ b/pkg/obiformats/fastaseq_read.go
@@ -219,6 +219,9 @@ func ParseFastaChunk(source string, ch FastxChunk) *obiiter.BioSequenceBatch {
 			if !is_end_of_line {
 				// Beginning of sequence
 				start = i
+				if C >= 'A' && C <= 'Z' {
+					ch.Bytes[current] = C + 'a' - 'A'
+				}
 				current = i + 1
 				state = 6
 			}