The distribution and abundance of mosquito larvae in a freshwater marsh were determined during a low water period in the late summer. Culex tarsalis larval abundance was significantly associated with the percent cover by Typha spp. root masses and Typha spp. stem density in 1-m2 quadrants. Larval mosquito abundance was not significantly related to bulrush (Scirpus californicus) stem density, position in the marsh (transect compass direction), position in the inundated vegetation on each transect or several physicochemical factors. As water levels decline seasonally, mosquito developmental sites in San Joaquin Marsh change from mats of decaying vegetation to cattail root masses.