We investigated the feeding ecology of the tidewater gobyEucyclogobius newberryi, a California estuarine fish that is federally listed as an endangered species. We compared the diet during three seasons (April, August, and November 1992) in three habitats (lagoon, creek, and marsh). Tidewater gobies (N= 331; standard lengths [SL], 15–46 mm) fed on benthic invertebrates, principally ostracods, chironomid larvae, and the gammarid amphipodCorophium spinicorne. Other prey taxa included polychaetes, oligochaetes, isopods, the gammarid amphipodEogammarus ramellus, copepods, mysids, and invertebrate eggs. Diet variability within samples was low, but diet varied across seasons and sometimes habitats. The tidewater goby appeared to be a specialist that can forage opportunistically in some circumstances. Comparisons between the diet and available resources in August indicated selectivity in the lagoon and marsh (albeit for different prey), but general feeding in the creek. Laboratory observations of feeding behavior revealed preferences forC. spinicorneand chironomid larvae (without the mucus tube) and suggested that substrate differences among the habitats could influence the diet by altering the mode of feeding (sifting sand in mouth versus plucking prey from gravel). Comparison of the diets ofjuveniles (<27 mm SL) and adults did not reveal an ontogenetic shift in diet. Analysis of the daily feeding pattern in November showed that juveniles fed at all hours whereas adults fed principally at night. The tidewater goby's feeding ecology suggests that it is adaptable to a variety of habitats. Fish from the marsh were substantially larger than those from other habitats, suggesting that marshes provide better opportunities for growth. The tidewater goby's diet overlaps with those of other California gobies, including two introduced species that are potential competitors and predators.