Larval amphibians (tadpoles) are an important link in aquatic food webs, as they can be highly abundant consumers and prey for a wide variety of predators. Most tadpoles are considered omnivores, predominately grazing on algae, detritus and macrophytes, though recent work has identified greater plasticity and breadth in diet than previously considered. We used gut content and stable isotope analysis (SIA) in a baseline study to determine the important dietary items (ingested material) and food sources (assimilated material) for tadpoles of two abundant generalist frog species in regulated floodplain wetlands of the Murrumbidgee River, south-east Australia. We identified a wide variety of dietary items in the gut contents, including whole microcrustaceans, filamentous algae and macrophytes. The composition of several ingested food items was correlated with their availability in each wetland. However, SIA identified biofilm as the food source most consistently assimilated across several wetlands, though microcrustaceans and algae contributed when abundant. Biofilm is likely the most important basal food item for tadpoles in floodplain wetlands because it is ubiquitous and has a high nutritional quality. Identifying important food sources is a crucial step towards developing management strategies for promoting tadpole recruitment in regulated wetlands.