A nutritional assay has been conducted with rainbow trout fry (Oncorhynchus mykiss) using a biomass of photosynthetic micro-organisms. The algal biomass was incorporated in the feed at increasing levels (12.5%, 25% and 50%) in order to verify the effects on survival (%), growth (length and mass) and carcass quality of the fish (skin color, water, protein and lipid content). Two commercial feeds (CF, as sold, and 0% A, reprocessed without algae) have been used as controls. After eight weeks at 10 degrees C, the growth has been significantly reduced in fish fed the feeds containing 25% and 50%,algae diets (P < 0.01). Moreover, an incorporation of algae higher than 12.5% in the feed led to a increase of water content (CF versus other diets; P < 0.01) and TL (CF versus 50% A; P < 0.01) (wet basis) content of carcasses. Survival was 100% in all treatment groups. Under the experimental conditions used, the results obtained show that a maximum of 12.5% of algal biomass can be incorporated in the feed for rainbow trout fry (O. mykiss) without negative consequences on growth and body content in lipids and energy of fish. (c) 2006 Elsevier Ltd. All rights reserved.