Studies with annual fish diet have shown that the generalist habit is a dominant strategy within this group. Cynopoecilus fulgens (Cyprinodontiformes: Rivulidae) is a restricted range species, endemic to a small area in Southern Brazil, and aspects of its biology and ecology are poorly known. The main goal of the study was to determine the diet of C. fulgens throughout its life cycle and compare it with the prey availability in Southern Brazil wetlands. Specifically, we analyzed samples collected in different stages of ontogenetic development (immature and adult fishes) in various year seasons. A total of 52 food items were found in the diet of C. fulgens, and invertebrates comprised 96.4% of the counts observed. The richness of consumed and available prey changed along with the seasons. The seasonal variation of the consumed prey richness was mainly related to increase in fish size along the annual cycle. Microcrustaceans represented 75.6%, 80.9%, and 68.4% of the consumed prey in autumn, winter and spring, respectively. Prey composition varied with the seasons and it was not correlated to prey availability. Diet selectivity was different among male, female and immature fish. Our study reported the feeding habits of Cynopoecilus fulgens to improve knowledge of the biological and ecological traits of annual killifishes in Southern Brazil wetlands, where more than 90% of the original wetlands have been lost due to agricultural expansion.