The Flatwoods Salamander (Ambystoma cingulatum) was listed as a federally threatened species because loss and alteration of adult and larval habitats caused population declines. Current management plans for this species must include maintenance of appropriate larval habitat. However, much of the natural history of this species is poorly known. Our objective was to evaluate the diet of A. cingulatum larvae. We examined stomach contents of 122 larvae collected from Florida and South Carolina. We identified and measured invertebrate prey and estimated biomass using length-mass relationships. Crustaceans, mostly isopods (Caecidotea), amphipods (Crangonyx), cyclopoid copepods, and cladocerans (primarily Simocephalus and other daphnids) were the most abundant prey. Isopods and amphipods also dominated stomach content biomass because of their relatively large size. Stomachs of small A. cingulatum larvae had significantly higher numbers and proportions of cladocerans and smaller numbers and proportions of isopods compared to larger larvae (P < 0.05). Smaller larvae also had significantly lower amounts and proportions of isopod biomass in their guts compared to large larvae (P < 0.05). We found Hedruris siredonis, a parasitic nematode, in ∼11% of stomachs examined; this is a new host record for this parasite. Our results indicate that larvae feed primarily on freshwater crustaceans, particularly isopods, copepods, cladocerans, and amphipods. Management activities that adversely impact freshwater crustacean communities in breeding habitats, such as pesticide use or introductions of predatory fish, could adversely impact remaining populations of this salamander.