The mesopelagic fish fauna in the Slope Sea off the Canadian continental shelf from 50° to 64°W is documented based on 10 surveys conducted in 1984–89. Species composition and relative species abundance by water mass (Labrador Slope Water [LSW], Warm Slope Water [WSW], and Gulf Stream), and their variation with season and fishing depth, are described. The most abundant species in collections was Ceratoscopelus maderensis, and this species dominated in WSW samples, but Benthosema glaciale was the predominant species in LSW samples. Gulf Stream samples were few and represented boundary conditions. Those species that were common in catches showed a seasonal variation in catch rate. Species composition varied with depth fished, deeper tows containing species, such as Cyclothone spp., that did not migrate to surface layers at night. The collections were comprised of at least 250 species. Although many of them have been reported from adjacent areas to the southwest, 35% of these species are new records for the sampled area, and 19 species are new records for the Slope Sea. Although a species accumulation curve indicates that more species would have been captured with additional sampling, it is likely that all species that occur regularly in the study area are represented in the species list.