Schistosomes utilise haemoglobin from ingested host erythrocytes as their main source of amino acids. Using reverse-transcriptase PCR, we detected transcripts encoding cathepsin D in eggs, miracidia, and adult male, female and mixed-sex Schistosoma japonicum. Using the synthetic fluorogenic peptide, o-aminobenzoyl-isoleucyl-glutamyl-phenylalanyl-p-nitro-phenylalanyl-a rgi nyl-leucine-NH2, and human haemoglobin as substrates, we detected cathepsin D-like aspartic protease activity at pH 3.6 in extracts of these developmental stages which was completely inhibited by the addition of l0 microM pepstatin. Using immunoblotting with rabbit antibodies raised against recombinant S. japonicum cathepsin D, we detected the aspartic protease in extracts of all developmental stages examined, although it appeared to be expressed at higher levels in the adult female schistosome. These results indicate that (almost) all stages of S. japonicum, express an aspartic protease. Moreover, they are consistent with the hypothesis that this enzyme plays a key role in maturing and adult schistosomes in the proteolysis of host haemoglobin from ingested erythrocytes.