The number of heterotrophic bacteria and vibrios in shoal for Bullacta exarata rearing fluctuated mainly within 1.25 x 10(4)-7.21 x 10(4) cfu.g-1 and 0-1.08 x 10(4) cfu.g-1, respectively. 173 strains of bacteria, mainly belong to 13 genera were isolated. The predominant genera were composed of Bacillus, Photobacterium, Pseudomonas, Vibrio and some genera of Enterobacteriaceae. The number of bacteria changed in line with the alteration of the temperature, and was especially affected by the organic matter content after the use of pesticide. The predominant genera such as Bacillus were closely related with the environmental deterioration of rearing shoal.