The competitive effects among tadpoles make the bullfrog a possible agent of native amphibian population declines at its introduction sites. Our aim was to analyze the dietary composition of an invasive population of Lithobates catesbeianus tadpoles and to evaluate the degree of dietary changes among activity months. We hypothesized that bullfrog tadpoles would exhibit temporal changes in diet, consistent with the variation of tadpole size and availability of resources. We collected 94 bullfrog tadpoles and identified 50 taxa in their digestive tracts. The items most frequently consumed were microalgae of the genera Navicula (frequency of occurrence [Fo] = 19) and Cymbella (Fo = 17). The trophic niche breadth was 8.13, which is considered moderate-high. The trophic habits of L. catesbeianus larvae changed seasonally, with the diets during October and December being numerically similar (Morisita's quantitative index = 0.86). The composition of the diet had a similarity of 63% (Jaccard's qualitative index) between August and October. The larvae of L. catesbeianus collected during the different months showed significant differences in their stages of development and body mass. On average, individuals were larger and more developed in the month of December, which was to be expected given that the abundance of prey items is greatest during the summer season. Our results show that L. catesbeianus tadpoles have a wide trophic niche with a diet that is not strongly selective; thus, the aquatic life-history stages play an important role in the structuring of invaded anuran larval communities.