Planktic foraminifera form shells that are preserved in ocean sediments and are used in a variety of paleoproxy and biostratigraphic applications. However, species-specific ecology can complicate the interpretation of planktic foraminifera-based proxies, and limited options exist for examining the ecology of extinct species. Here we apply test-bound compound-specific stable isotopes of amino acids (CSI-AA) to examine the trophic ecology of extant planktic foraminifera. We measure CSI-AA in planktic foraminifera shells collected in sediment traps from the Santa Barbara Basin, CA, specifically the three most abundant species in this region: <em>Globigerina bulloides</em>, <em>Neogloboquadrina incompta</em>, and <em>Turborotalita quinqueloba</em>. The nitrogen CSI-AA of all three species suggest that planktic foraminifera have metazoan-like metabolisms, and that trophic position estimates using CSI-AA are appropriate for planktic foraminifera. All three species had trophic positions near 2 (primary consumer), with no evidence for mixotrophy or photosymbionts. Carbon CSI-AA, in combination with a Bayesian stable isotope mixing model, indicates that the three species occupied separate niches based on diet. <em>Globigerina bulloides</em> fed opportunistically on all groups of phytoplankton available in Santa Barbara Basin, adjusting its diet with seasonal changes in phytoplankton assemblage. <em>Turborotalita quinqueloba</em> specialized in diatoms and heterotrophic bacteria. <em>Neogloboquadrina incompta</em> consumed heterotrophic bacteria and some phytoplankton. Our results align with the current understanding of each species’ ecology while further defining their niches. Our findings suggest that CSI-AA is a promising tool for understanding the trophic ecology of planktic foraminifera, and we make recommendations for future applications of CSI-AA to fossil specimens.