The importance of nutritional components on growth and body composition outcomes has been demonstrated in multiple model organisms. Although zebrafish (Danio rerio) have an established role in research laboratories for its utility in understanding developmental biology and genetics, the influence of diet composition on basic growth outcomes is less well demonstrated. In the current study, four protein sources were tested in isolation using isonitrogenous diets or combined using a defined lab diet. Fish (n≈60/group) were group housed (n≤10 fish/1.8 L tank) and fed ad libitum three times daily for 12 weeks. Fish were assessed for effects on length, body weight, and body composition (lean and fat mass). Individuals fed wheat gluten protein were significantly shorter in length, with significantly lower body weight and lean mass in both male and female fish, although percent body fat was high compared with other diets. Casein-fed fish similarly had significantly reduced body length, body weight, and lean and fat mass in both male and female fish, with a low percent body fat compared with other diets (leanest). Fish protein hydrolysate-fed fish had significantly lower lean mass and a high percent body fat, whereas soy protein isolate diet performed similarly to a mixed-protein control diet for all measured outcomes. These results suggest that the protein source, with accompanying amino acid ratios or additional protein source differences, has a significant impact on growth and body composition outcomes in zebrafish when fed in a semipurified, defined diet background. © Copyright 2013, Mary Ann Liebert, Inc.