–Juvenile pen shells Atrina maura were fed for 21 d with equal daily rations, equivalent to 6.5% of their live weight, of monoalgal cultures of Tetraselmis suecica, Isochrysis galbana, Chaetoceros muelleri, Chaetoceros calcitrans, and Thalassiosira pseudonana. Pen shell survival was 100% in all cases. All microalgae diets produced shell growth, and live and ash‐free dry weight increases. However, the two Chaetoceros strains gave significantly better results, with shell height increases of 4.7 to 2.9 mm, live weight gains of 0.5 to 0.3 g, and final ash‐free dry weights of 0.16 to 0.14 g after 3 wk. These results are not related to the algae gross biochemical composition, suggesting that other factors are involved in their nutritional value for this mollusc species.