Little is known about the ecology and life history of the federally endangered Point Arena mountain beaver (Aplodontia rufa nigra). The distribution of this primitive burrowing rodent is disjunct from the balance of the species' range and occurs in a unique maritime environment of coastal grasslands and forests. Fundamental to protecting this taxon is knowledge of its breeding season and other reproductive characteristics. This information is necessary so that ground-disturbing activities near burrow systems can be scheduled to avoid the breeding season. We examined the reproductive status of 38 individuals, captured 150 times, from June 2004–July 2005 (excluding 01 February–01 May when we were prohibited from trapping). We used changes in vaginal cytology to monitor estrus and we evaluated male breeding readiness by determining whether testes were scrotal. The earliest onset of estrus occurred on 02 December and it was detected until the trapping prohibition period began on 01 February. The first males had scrotal testes somewhat earlier, in late November, and most males were still in this condition when trapping prohibition began. These data suggest an earlier onset of the breeding season than described for other subspecies. All females and a majority of males (70%) examined in December and January had evidence of reproductive condition. No evidence of pregnancy was observed during the study period, but based on allometric equations developed elsewhere, and applied to the weights of juveniles we captured, parturition was estimated to occur in early- to mid-April.