In the last decade the novel receptor-like protein tyrosine phosphatase was identified and termed osteotesticular tyrosine phosphatase (OST-PTP) due to its restricted expression in bone and testis. OST-PTP expression is regulated during osteoblast differentiation and it shows stage specific expression in the testis. Confined OST-PTP expression in the basal compartment of the seminiferous tubules suggests that this protein may be a good candidate for a rodent germ stem cell marker. To test this hypothesis we determined the exact pattern of OST-PTP expression in the rodent testis.Adult mouse and rat paraffinized testis sections were subjected to in situ hybridization using riboprobes against the receptor and catalytic domains of the protein OST-PTP.OST-PTP testicular expression in rodents is not confined to the spermatogonia, as previously suggested, but is also present in Sertoli cells in a stage independent pattern. This finding excludes the hypothesis that OST-PTP is a germ stem cell identification marker in rodents. In addition, we report the identification of a testicular OST-PTP isoform lacking part of a catalytic domain that is widely expressed during spermatogenesis in all cell types.This finding implies tight control over OST-PTP expression in the testis, which in turn suggests an important role for OST-PTP and its isoform in male germ cell differentiation.