A DNA clone representing a 0.4 kb degenerative repeat has been isolated. The DNA sequence is present only in the genome of female Schistosoma mansoni at different stages of the life cycle, at a frequency of approximately 75 copies per adult female genome. The sequence is not expressed and probably represents satellite DNA in the heterochromatin region of the W chromosome. It is demonstrated that the DNA clone may be used for the rapid determination of the sex of cercariae without the need for DNA isolation or Southern blotting.