Diet, reproductive patterns, and sexual dimorphism are described of Chironius fuscus from Brazilian Amazonia based on the analysis of museum specimens (n = 120 for diet and reproduction; n = 333 for dimorphism). The diet was composed primarily of anurans. No relationship was found between snake body size and prey type, and the species showed no preference for direction of prey ingestion, probably due to the consumption of a prey type that does not present injury risk to the snake. No differences in diet composition were found between males and females or adults and juveniles. Many breeding females had prey in their digestive tracts. Males reach sexual maturity atsmaller size than females and no evidence of seasonal breeding was found. We observed 1–6 secondary follicles ( = 3.3) and 3-6 eggs ( = 4.3). The species is sexually dimorphic, with males being significantly longer and having a higher number of ventral scales, whereas females have significantly longer tails, larger eyes, and a higher number of subcaudal scales. This species differs from most other colubrids, especially arboreal species, which tend not to show sexual dimorphism. No significant difference between sexes was found in head length or width.