The blood feeding patterns of Anopheles freeborni Aitken and Culex tarsalis Coquillett were studied, and the effects of host availability on these patterns were assessed in four different habitats within a northern California rice agroecosystem. Resting mosquitoes were collected from June to September of 1991 and 1992. The source of mosquito blood meals was identified with the modified precipitin test. Anopheles freeborni exhibited a 'specialized' (fixed) blood feeding pattern, predominantly (99% of the time) feeding on mammalian hosts; leporids and bovids were the major hosts, while equines, suids, and other mammals were minor hosts. Culex tarsalis exhibited a more 'generalized' (catholic) blood feeding pattern, taking blood meals from both birds and mammals at a ratio of 3:1 with Passeriformes being the most fed upon host group. Human blood indices were under 3% for both mosquito species, and multiple blood meals were estimated at less than 2%. The host feeding patterns for both mosquito species differed among the four (riparian, rice, pasture and mixed) habitats. The host feeding pattern for C. tarsalis reflected the distribution of both mammalian and avian hosts available. On the contrary, the host feeding patterns for A. freeborni reflected the distribution of mammalian but not the available avian hosts. Overall, host availability may be an important determinant of population size of some mosquito taxa (e.g. A. freeborni) than others (e.g. C. tarsalis) in rice culture agroecosystems.