The karyotypes of Reithrodontomys megalotis longicaudus, R. raviventris halicoetes and R. r. raviventris in the San Francisco Bay region were studied to understand better the taxonomic relationships of the three taxa. Reithrodontomys megalotis has 44 chromosomes whereas the two endemic suspecies of raviventris have 38. The karyotypes of R. r. halicoetes and R. r. raviventris are different enough to suggest that the two are in the terminal stages of speciation.