Different sources of microbial biomass have drawn attention as novel ingredients for aquaculture feeds. In the present study, isotopic measurements were applied to determine the contribution of dietary nitrogen supplied by two sources of microalgal biomass and fish meal, to the growth of shrimp Penaeus vannamei. Microalgae Schizochytrium sp. and Grammatophora sp. were isolated from the Sea of Cortez and massively cultured to obtain sufficient biomass. Experimental diets were formulated with low levels of microalgal biomass replacing 5 and 10% of fish meal nitrogen. Nitrogen stable isotope values were determined in ingredients, diets, and shrimp to estimate the relative contributions of the dietary nitrogen and dry matter supplied by these ingredients to the somatic growth. At the end of a feeding trial, significant differences were observed in mean final weight gain. Dietary nitrogen contributions from microalgae were similar to established dietary proportions, but when estimated on a dry matter basis, nutritional contributions were different for a diet containing 10% of Schizochytrium, which contributed 24% of dry matter to growth. Results demonstrate that low dietary inclusion levels of microalgal biomass elicit similar or higher growth rates than diets based on a fish meal only. Isotopic data indicated that microalgae actually contributed protein to tissue accretion.