The main purpose of this work was to evaluate the colouring effect of carotenoids present in dry biomass obtained from stressed cells of Chlorella vulgaris. Total carotenoids represented only 0.2 % of dry algal biomass and consisted mainly of canthaxanthin, lutein, astaxanthin and their esters. -A rainbow trout feeding trial was conducted in order to investigate the effects of dietary algal incorporation as a pigment source, in comparison to that obtained with synthetic pigments, both astaxanthin and canthaxanthin. -The colour intensities measured in muscle were compared, in each case, with those which constitute the Salmonids Roche Colour Card, and proved similar after both 3 and 6 weeks feeding, with a significant increase detected, but at the end of experiment (9 weeks) the increase was shown only in spectrophotometric (quantitative) results. ChlorelIa vulgaris biomass, which is storable without any special precautions, thus appears to be a promising source of carotenoids to use in commercial fish finishing diets, quite comparable in efficiency with the existing synthetic pigments. Algal biomass micronization, which was also tried out, was not found to be advantageous for the efficiency of colouring, no evidence having been found of any difference in its absorption and in the distribution of pigment through muscle, even if a 150-g rainbow trout, which has a rather short digestive tract, was used.