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In a completely randomized experiment, the effect of Daphnia magna meal on growth performance and carcass composition of rainbow trout (Oncorhynchus mykiss) larvae (body weight 0.538±0.197 g) was evaluated for a period of 60 days. Five diets with the same protein content (crude protein 52.70%) were prepared, including two diets containing fermented daphnia meal T₁ (20%), T₂ (30%), two diets containing raw meal T₃ (20%) and T₄ (30%), and a basal diet without daphnia meal as the control. The bacterial species used for fermenting included five commercial probiotic species. Fermented daphnia meal brought about significantly higher growth and the lowest feed conversion ratios, viz. T₁ (1.27 ± 0.35) and T₂ (1.31 ± 0.46), in comparison with T₃ (1.38 ± 0.37), T₄ (1.41 ± 0.42) and the control (1.84 ± 0.69) (p<0.05). The result of carcass composition analyzes showed that the highest protein percentage (68.23 ± 0.48%) was in T₂ and the highest percentage of fat and energy (11.26 ± 0.50% , 4761.63 ±48.88 Cal g^-1) were in T₄ (p<0.05). The overall results of this experiment indicated that daphnia could be considered as a suitable partial protein source in rainbow trout larval feed.