The in situ polymerase chain reaction (PCR) results revealed that mouse type A and type C retroviral sequences were transmitted horizontally from the host to schistosomes. The signals to these retroviral sequences were observed in the nuclei of the mesenchymal and reproductive cells of 8-week Schistosoma japonicum. These signals were also detected in the nuclei of the mesenchymal and reproductive cells and in the cytoplasm of the tegumental tubercles of 24-week S. mansoni. Furthermore, mouse type A retroviral sequence was detected in the DNA extracted from the cercariae of both species. However, mouse type C retroviral sequence and mouse type 2 Alu sequence (B2) were difficult to detect in the cercarial DNA of either species. These findings may indicate that some host sequences are propagated in the schistosome progeny, that is to say, not only horizontal but also vertical transfer of the host gene may occur in schistosomes.