We quantified frog phenology and microhabitat use of the native California red-legged frog (Rana draytonii) and introduced bullfrog (Rana catesbeiana) in an 11-ha seasonal marsh, Sonoma County, California. Logistic regression showed that both species selected habitats nonrandomly from among the available habitats in the marsh. As adults, the two species overlapped in their habitat use, selecting dead spikerush in winter and spring, and aquatic buttercup in summer. Although the model emphasized overlap in frog habitats, there was more separation in habitat use between species during winter than other seasons when few bullfrogs were active (i.e., most bullfrogs hibernating). The egg-laying habitats and seasons differed dramatically between the two species. Red-legged frogs bred in winter almost exclusively in shallow dead spikerush and bullfrogs in spring and summer in deeper areas with dense cover, predominantly smartweed. Breeding periods of red-legged frogs and bullfrogs were separated by 10 wk, which coincided with peak adult abundances. We suggest that the separate reproductive seasons may reduce competition and predation by bullfrogs on red-legged frogs, allowing for coexistence. Furthermore, the marsh's late-season drying limits metamorphosis of bullfrog tadpoles, which usually require permanent water. The marsh's seasonal hydrologic pattern offers a model for habitat in which the native red-legged frog may persist despite negative interactions with the introduced bullfrog.