Owing to the high cost of microalgal production for bivalve culture, a need for alternatives to live microalgal diets exists, particularly in facilities where space and personnel required for live microalgal culture are limited. This study examined the feasibility of utilizing commercially available microalgal concentrates in place of or in addition to live microalgae as a feed for post set northern quahogs (= hard clams), Mercenaria mercenaria and sunray Venus clams, Macrocallista nimbosa. Clams were fed one of six dietary treatments for 6 wk, and growth, survival, and fatty acid (FA) profiles were evaluated. Treatments included two live microalgal diets Tisochrysis lutea and T. lutea + Chaetoceros neogracile, which served as controls for complete or partial replacements with two commercially available concentrate products, ISO 1800 and Shellfish Diet 1800. The production of both clam species was negatively impacted compared with the live microalgae controls when only concentrates were fed. Survival decreased by 29–33% for hard clams and 42–62% for sunrays; a similar decrease in the growth rate (25–34%, 42–62%) was seen for hard clams and sunrays, respectively. Survival was not affected by feeding the partial replacement diets; however, the growth of sunrays was negatively affected (32–38% decrease) compared with live microalgae controls. Clam tissue FA profiles generally reflected the FA profiles of the dietary treatment fed, yet production was decreased with complete replacement diets, suggesting nonnutritional factors as the cause for the poorer performance seen. These results suggest that a live microalgal diet may be able to be sufficiently replaced with up to 50% of the microalgae concentrates tested without affecting the production of hard clams. For both post set hard clams and sunrays, production will be decreased if the two concentrates used in this study are utilized exclusively.