The amphipod sibling species Ampelisca vadorum and A. abdita have an overwintering generation and a summer generation. Breeding begins above 8 C. In A. abdita, breeding is probably correlated with the full moon. Curves of growth in length are linear, except where growth slows in winter. Length–weight curves are exponential, and A. vadorum is slightly heavier than A. abdita at any given length. The slope of the length–weight curves is the same in both species. Growth in weight is not isometric; weight increases less rapidly than does length. In some allometric features the two species stop growth at different points on the same line, but other morphological features develop out of phase due to genetic differences in patterns of development. The two species differ in a number of biological features including egg size, metabolic contribution in egg production, relative gill area, and probably egg number. Reported "intersexes" are actually subadult males. The sex ratio in both species is always 1:1. Gonad growth and somatic growth have different temperature limits.Tubes, tube building, and feeding are described. Three major functional groups of ampeliscids show marked morphological adaptations to feeding type and sediment. Free swimming occurs at times of breeding, ensuring fertilization of eggs and dispersal of young to new and favourable environments.Ampelisca vadorum occurs on course sand, A. abdita on fine sand to mud. Range, temperature, and depth preferences of these and other North American Ampelisca species are outlined. The fauna associated with A. vadorum and A. abdita is described. The unique microtopography of Ampelisca areas affects the nature of the association. Ampelisca abdita is successful in a crowded environment because it grows fast and breeds early. Ecological, life history, and taxonomic evidence all indicate that A. vadorum and A. abdita are sibling species.