Morphometry, reproductive biology, diet, and epizoite infestation of Ovalipes stephensoni (Brachyura: Portunidae) are described. The results are based on observations of 347 specimens collected by demersal trawl on the continental shelf of the eastern United States from 13-198 m, between 28° and 37°N, and on 26 specimens from the National Museum of Natural History collections in Washington, D.C. All seven morphometric relationships observed (carapace length, body depth, propodal length, width and depth of crushing chela, and abdominal width) exhibited allometric growth to some extent. Sexual dimorphism was demonstrated for chelar propodal length and abdominal width. Prepubertal and pubertal molts were inferred from two changes in allometry of the male chelar propodal length and female abdominal width. Allometric maturity was correlated with gonad, pleopod, and gonopore development. Males reach full maturity at a carapace width of 61 mm; females complete maturation by the time they reach 51 mm in width. Ovalipes stephensoni is heterochelic and heterodontic with the crushing claw occurring on the right side in 91.6% of the specimens observed. There is little evidence that cheliped laterality changes with age (size). Although this species is an opportunistic carnivore, bivalve mollusks, cephalopods, and small crustaceans comprised the bulk of the diet. Relatively heavy infestations of lepadid barnacles, Octolasmis muelleri, were found in the branchial chamber. Lesions of the ciliated protozoan Synophrya hypertrophica were present in the gill lamellae.