17 male-female pairs of California mice (Peromyscus californicus) were observed in a total of 60 tests of copulatory behavior, each continued to a satiety criterion of 30 min with no copulations. California mice showed a pattern with no functional lock, intra-vaginal thrusting, ejaculation possible on single insertions, and multiple ejaculations. The mean ejaculation frequency was 1.7. It is suggested that the evolution of copulatory behavior in hystricomorph and myomorph rodents has been less divergent than had been earlier proposed.