ABSTRACT: How an organism’s phenotype responds to both its biotic and abiotic environment is a complex interplay of selection pressures and adaptive tradeoffs. Bathymetric patterns of body size in deep-sea organisms should also reflect both ecological role and taxon-specific constraints, as exemplified in a variety of recent studies. Here, we examine bathymetric size clines in deep-sea ectoparasites, a group that has received little attention in the literature compared to other deep-sea groups. Specifically, we focus on body size in 3 families (21 species) of ptenoglossate gastropods from the deep western North Atlantic, conducting analyses both within and among species. Both quantile and linear regression models yielded non-significant relationships for body size and depth for the 3 most abundant species. Two of the 3 families exhibited positive size–depth relationships, but only in mean size. The findings indicate that resource availability/host size may control parasite density, but dislodgement and predation risk may set a hard upper boundary on body size in deep-sea ectoparasites. Moreover, this study stresses the necessity of understanding the ecological role of species in investigating body size trends.