Geographical and vertical distributions, reproduction, growth and maturity, food and feeding of Meganyctiphanes norvegica, Thysanoessa raschii, T. inermis, and T. longicaudata in the Gulf of St. Lawrence were studied, based on plankton samples from 10 cruises, largely from 2 consecutive yr. Adults of the first three species occurred most abundantly in the western Gulf and their larvae in the Magdalen Shallows. By contrast, T. longicaudata, an oceanic species, occurred mainly in the eastern Gulf year-round. All four species reached reproductive maturity at 1 yr of age; all but T. longicaudata populations contained at least two breeding year-classes. The three Thysanoessa species started spawning in April at the time of the phytoplankton bloom; M. norvegica was a summer breeder. All four species were omnivorous, but M. norvegica and T. longicaudata stomachs contained relatively more animal matter than those of the other two. These findings were supported by studies on the morphology of feeding appendages.Thysanoessa longicaudata was differentiated from the other species mainly on the basis of distribution patterns, and M. norvegica from the remaining two mainly on the basis of differences in feeding and reproductive season. In contrast to relatively clear resource partitioning in five of the six species pairs, only relatively small differences in the stomach content composition in winter samples, the spacing of setules, and the length of the breeding season differentiated T. inermis from T. raschii.