A trial was conducted to test the effect of replacement of fishmeal by yeast, in isonitrogenous (48% CP) and isoenergetic (22 MJ kg(-1)) diets for gilthead sea bream (Spar-us aurata) with an initial average weight of 90 +/- 4 g. Diets were formulated to include 0% (control group), 10% (group I) and 20% (group II) from replacement of fish meal by yeast. Each diet was distributed by hand to satiation to triplicate groups of 25 fish per tank (1 m(3)) and the growth trial lasted 12 weeks. Fish were adapted to study tanks and fed with control diet for one week. Fecal collection was began in the second week and samples were stocked in deep freeze (-20 degrees C). Live weights of fish at the end of trial period were 140 +/- 11 g, 154 +/- 3 g and 157 +/- 8 for control group, group I and II, respectively. Growth rate and protein, lipid and cellulose digestibility were insignificant different among groups (p>0.01). Plasma glucose was found significant (p<0.01) but alkaline phosphates, Blood Urea Nitrogen (BUN), serum protein, cholesterol, triglyceride, albumin, amylase, GOT and GPT were not significant (p>0.01).