The variation of chemical environmental factors in the water ecosystem , bacterial population and cultured organisms in the freshwater fishponds was studied after applying salt2resistant Rhodospirillaceae bacteria to culture Ictalurus punctatus and Pengze crucian. The results showed that the NH4 + -N ,NO2 --N ,NO3 --N and COD content decreased and dissolved oxygen and pH increased in the water ecosystem after sprinkling Rho. capsulata in the fishponds. The change of bacterial population and quantity was as follow : The amount of photosynthetic bacteria , nitrous bacteria and nitric bacteria rose 3 -50 (majority 15 -20 times ) , 3 and 4 -5 times respectively. The amount of major heterotrophic bacteria Enterobacteriaceae rose 9. 4 % and Aeromonas reduced 7. 5 % in the Ictalurus punctatus fishponds ; Pseudomonas reduced 16 % and Enterobacteriaceae rose 21 % in the Pengze crucian fishponds. The cultured effect of the cultured organism was that the average increasing rate of weight of every Icthlurus punctatus fry and Pengze crucian fry rose 15. 7 % and 12. 8 % respectively.