The ecology of the water snake, Grayia smythii (Reptilia: Colubridae) occurring in a seasonal rainforest swamp of the Niger Delta (southern Nigeria) was investigated between December 1998 and March 2000. Females and males were similar in body sizes (SVL) and head sizes, but males had tails significantly longer than females. The diet was constituted only by frogs and fish. The major prey type was Xenopus tropicalis, followed by Tilapia sp. and Clarias sp. Adult sex-ratio was 1:1. Sloughing of skin and ovipositions occurred in dry season, in the humid enclosure of buttress roots amongst leaf litters. Fecundity ranged from 8 to 14 eggs per female, with a mean of 10 eggs (SD = 1.8). The smallest gravid female was 78.2 cm SVL. Eggs were laid in batches of three to four eggs at a site, in at least two to three different sites. The size of the eggs averaged 3.1 cm in length, 2.1 cm in width, and had a fresh weight ranging from 18.2 to 22.1 g. Maternal size influenced significantly the number of eggs produced by female, but not their average size. There was a statistically significant negative correlation between number of eggs and mean egg size. Predators of this snake at the study area were herons and fishermen.