The immunostimulant potential of the whole fungus Mucor circinelloides administered in the diet to gilthead seabream (Sparus aurata L.) was studied. Three different lyophilised strains were used: the wild-type (R7B) and two mutant strains in the carotene-production pathway which are enriched in lycopene (MU224) or beta-carotene (T31). Fish were fed diets containing 0 (control) or 10 g of one of the whole M. circinelloides strains per kg feed. After 2, 4 or 6 weeks of treatment the growth rate as well as humoral (lysozyme activity) and cellular (phagocytosis and cytotoxicity) immune responses were determined. The specific growth rate increased slightly with all the M. circinelloides-supplemented diets. Serum lysozyme activity increased slightly (P>0.05) in fish fed the mutant strain-supplemented diets. Of the cellular responses, phagocytosis significantly increased after 6 weeks, in fish fed the wild-type strain-supplemented diets while cytotoxicity increased after 4 weeks in fish fed the beta-carotene-enriched strain-supplemented diet. The immune responses were increased to some degree by diets containing whole M. circinelloides strains. These results are discussed in the light of the on-going search for new microorganisms, wild or mutant-type, for use as immunostimulants in fish farming.