Moreover, bacteria, particularly members of the genus Bacillus secreted a wide range of exoenzymes ( Moriarty, 1996 and Moriarty, 1998). We could not distinguish between activity due to enzymes synthesized by the shrimp larvae and that due to enzymes synthesized by the probiotic strains colonized in the larvae digestive tract. However, the exogenous enzymes produced by the probiotic would represent, at most, only a small contribution to the total enzyme activity of the gut ( Ding et al., 2004 and Ziaei-Nejad et al., 2006), and the presence of the probiotic might stimulate the production of endogenous enzymes by the shrimp larvae. The observed increases in specific activities of digestive enzymes in probiotic treatments might have led to enhanced digestion and increased absorption of food, which in turn contributed to the improved survival in P. vannamei.