To determine whether shifts occurred in the food web associated with upwelling at the Brazilian western boundary South Atlantic Central Water (SACW), we measured isotopic composition (?15N and ?13C) of 38 fish and 3 squid species from the Cabo Frio food web. Sampling was performed during the austral summer upwelling period (February 2002) and the austral winter non-upwelling period (July 2001). Mean lipid-corrected ?13 ?13Ccor) values ranged from -20.17 to -15.02‰, and mean ?15N ranged from 10.28 to 16.08‰. An analysis of covariance performed using length as a covariate provided evidence of seasonal effects on the mean fish ?13Ccor and ?15N, with higher values during the non-upwelling period than during the upwelling period. These effects are most likely related to biophysical processes that occurred 1 to 6 mo earlier. The data furnish circumstantial evidence of a link between the dynamics of the SACW and the trophic structure of Cabo Frio. The trophic level of the species of fish and squids ranged between the theoretical trophic levels of 3.6 and 5.1, but there were no discrete steps, rather a continuous gradient in isotopic values. This suggests that these food webs are unstructured and the component species have mixed diets.