A mouse cDNA encoding a non-receptor-type phosphotyrosine phosphatase (PTP; EC 3.1.3.48) has been isolated. The 1570-base-pair cDNA contains a single open reading frame that predicts a 382-amino acid protein with Mr 44,640. The nucleic acid and amino acid sequences are homologous to those of a previously described human T-cell PTP [Cool, D. E., Tonks, N. K., Charbonneau, H., Walsh, K. A., Fischer, E. H. & Krebs, E. G. (1989) Proc. Natl. Acad. Sci. USA 86, 5257-5261]; however, the mouse and human 3' sequences diverge and predict markedly different protein carboxyl termini. The mouse PTP gene is expressed as a 1.9-kilobase message in several stages of murine embryonic development and in a variety of adult tissues. An additional 1.3-kilobase message was found to be expressed specifically in testes. Finally, we report the isolation of a human T-cell PTP cDNA containing a 3' end sequence homologous to the mouse PTP.