Culture of the sea scallop, Placopecten magellanicus, is constrained by a reliable supply of high-quality postlarvae, yet little is known about the diets and essential nutrients required to maximize growth and survival during these vulnerable stages. Therefore, post-settlement sea scallops were exposed to binary microalgal diets consisting of a flagellate: Pavlova lutheri, Pavlova sp. (Pav, CCMP 459) or Tetraselmis striata (Plat-P) and a diatom: Chaetoceros muelleri, Thalassiosira weissflogii or Fragilaria familica for 28–30 days. The combination Pav 459/C. muelleri provided a superior diet for sea scallop postlarvae, yielding a growth rate of up to 28 Âµm day-1. However, when these algae were offered singly, a 32% (Pav 459) and 64% (C. muelleri) decrease in growth rates was observed, indicating that both species made a significant contribution to the success of the mixed diet. The two species are characterized by unique signatures of n-6 polyunsaturated fatty acids (PUFAs): C. muelleri has high levels of arachidonic acid (AA), and Pav 459 (in contrast to P. lutheri) has high levels of n-6 docosapentaenoic acid (DPA). DPA was selectively incorporated into tissues of scallops fed all binary diets except the one rich in DPA (Pav 459/C. muelleri). These results, coupled with a marked increase in n-6/n-3 ratios between the diet and tissues, provide evidence that n-6 PUFAs may play an important and previously underestimated role in scallop nutrition. The diet of T. striata and C. muelleri, which yielded the lowest growth rate (8.1 Âµm day-1) contained dietary docosahexaenoic acid (DHA; 22:6n-3) concentrations 4.5–7Ã舒 lower than any other binary treatment. Low DHA levels may thus explain the poor performance of this diet. Tissue protein and triacylglycerol concentrations reflected diet performance, with higher values generally associated with more successful diets. This work suggests that DHA, as well as the n-6 PUFAs AA and DPA, may be essential for optimizing growth of sea scallop postlarvae.