Echinanthera undulata is an endemic dipsadid snake from the Atlantic forest in southeastern Brazil. We gathered data on body size, food habits, reproductive cycles, and seasonal abundance of 240 preserved specimens. Adult females are larger in snout-vent length and have shorter tails than males. Males and females do not differ in relative head and eye size. Echinanthera undulata feed mainly on small leptodactylid frogs on the leaf litter. Apparently, its long tail is used to facilitate the capture of prey in this substrate (using poking behavior, leading the anuran to flee in the leaf litter). Reproductive cycle is seasonal in both females and males. Females showed vitellogenic follicles and oviductal eggs mainly at the onset of the rainy season, and births may occur mainly at the onset of the dry season. Fecundity ranged from one to 19 eggs. Testes volume was significantly larger at the end of the rainy season, but the deferent ducts diameter did not differ significantly throughout the year. Variation in seasonal abundance may be related mainly to reproductive events in females and males.