The Dice Snake (Natrix tessellata) is an aquatic colubrid that is known for having a basically piscivorous diet and a female-biased sexual size dimorphism (larger females). The feeding habits of three populations of N. tessellata from Mediterranean streams in central Italy were studied. The three streams differed in terms of their water regimes, one being seasonal and two being perennial. More than 2,000 snake individuals were handled for food items, and 1,200 prey items were collected. The percent of fed snakes was significantly influenced by the study area and by the interaction term between study area and season (i.e., by the hydric regime). Fishes accounted for over 90% of the snake diets at all study areas, the most common prey species being cyprinids. Overall, the three snake populations had similar taxonomic diet composition. There was a significant ontogenetic change in diets in all populations. There were also significant intersexual differences in prey composition, with the females taking more Scardinius erythrophtalmus fishes and more anurans, and the males more Leuciscus s. muticellus. The various fish species found at the three study areas (15 species) were classified in terms of six independent ecological characteristics in order to investigate what factors make a given prey species suited or not for Dice Snakes. It appeared that N. tessellata selected mainly fish species that were gregarious, small to medium sized, active foraging, diurnal, and not using mud as preferred substratum. We also found that the seasonal desiccation of the stream-bed of one study area caused a shift in the diet composition of these snakes, with males and juveniles being the categories where the seasonal dietary shift was more evident. Males preyed especially on the fishes S. erythrophtalmus and Leuciscus cephalus, and the juveniles preyed upon Leuciscus s. muticellus, whereas in spring their primary prey was Alburnus a. alborella.