ABSTRACT: The gulf menhaden Brevoortia patronus is an important species of the coastal ecosystem and the target of the largest fishery by landings in the Gulf of Mexico. Menhaden species forage on a variety of plankton and detritus and, by grazing plankton stocks, may provide an important regulatory ecosystem service by reducing organic material loadings. This study used stable isotope analysis to examine the spatial, temporal, and ontogenetic dynamics of food selectivity and trophic role observed in gulf menhaden. The most important dietary item for juvenile (<100 mm total length) fish was phytoplankton (74.0% dietary composition), while that of sub-adults (100-200 mm) and adults (>200 mm) was zooplankton (61.6% for sub-adults and 52.4% for adults). Juvenile fish also utilized detritus when present in the water column, and their diet was more varied among individuals than sub-adult and adult age classes. Juveniles occupied a trophic level approximately one step lower (2.65 ± 0.31; mean ± SE) than sub-adults (3.50 ± 0.21) and adults (3.39 ± 0.19). Spatial dietary variation was related to known ontogenetic habitat shifts (i.e. onshore to offshore stratification of size classes), while temporal variation was minimal, especially in the larger size classes. Since the fishery largely targets age 1+ fish (sub-adults and adults), these results suggest that if overfishing occurs to the extent that it impacts recruitment, it may decrease the resiliency of the inshore Gulf of Mexico ecosystem to eutrophication by decreasing the abundance of juvenile fish seasonally present in this environment.