Large-scale, open, microalgal feed cultures for hatchery and nursery production of marine invertebrates inevitably becomes contaminated with various microbes that can affect productivity and usability of the harvested biomass. In the Greenhouse for Research on Algal Mass Production Systems (GRAMPS) at the NMFS Laboratory in Milford, CT, cultures of Tetraselmis chui (PLY429) often become contaminated with a cyanobacterium; preliminary observations suggested that juvenile bay scallops, Argopecten irradians irradians showed reduced performance when the feed culture became contaminated with this cyanobacterium. We isolated a cyanobacterium from a contaminated culture of PLY429 and conducted a feeding study to determine if this isolate affects survival and growth of juvenile bay scallops, either alone or in combination with PLY429, thereby simulating feeding of a contaminated culture. Bay scallops were given a diet of either 100% PLY429, 50% PLY429 with 50% cyanobacteria, 100% cyanobacteria, or starved. There was 100% mortality of bay scallops by week 3 when they were starved, with a significant difference in survival between diets (P < 0.01). At 6 wk the scallops fed only the cyanobacterium had 63% survival, 93% survived in the mixed diet, and 98% survived when fed 100% PLY429. The net growth of bay scallops on the different diets was also significantly different (P < 0.01) with scallops fed 100% PLY429 having the highest shell-growth rate of 198-μm scallop⁻¹ d⁻¹, and growth rates of 82-μm scallop⁻¹ d⁻¹ on the mixed diet, and 65-μm scallop⁻¹ d⁻¹ for the cyanobacterial diet. These findings suggest that the cyanobacterium will not cause instant mortality, but it will not support sustained survival and growth over time scales of weeks.