© 2018 European Association for Aquatic Mammals. Determining the trophic position and trophic width of animals within food webs is essential to defining their ecological role. California sea lions (CSLs; Zalophus californianus) are widely distributed along the dynamic coast of the northeast Pacific; thus, their diet is shaped by the unique setting and environmental conditions of each colony. Our goal was to determine the trophic position and isotopic niche of CSLs from two colonies in Mexico in distinct environments (Gulf of California vs west coast of the Baja California peninsula) by examining the carbon and nitrogen isotopic signatures in vibrissae. The Gulf of California is a marginal sea where primary productivity is based on seasonal upwelling and intense tidal mixing. In contrast, primary productivity on the west coast of the Baja California peninsula depends on seasonal upwelling as well as the productive California Current System. One mystacial vibrissa was removed from each CSL captured on San Esteban Island (SEI) (N = 10) in the Gulf of California during the 2011 breeding season, and on Santa Margarita Island (SMI) (N= 16) on the Pacific coast of the Baja California peninsula during the 2012-2013 breeding season. Bayesian techniques were used to determine the trophic niche and trophic position. The trophic niche was wider at SMI than at SEI as more habitats are available to female CSLs at the former (e.g., coastal, pelagic, and lagoonal). However, despite the wider trophic niche and array of habitat types, the trophic level was the same for both colonies, suggesting that CSLs may maintain their trophic position across variable ecosystems. The accurate evaluation of the geographic variation in the trophic position of female CSLs allows us to better understand how these marine mammals utilize the distinct resources available in the habitats they occupy.