Abstract: A study was carried out on species and specific composition of aquatic microorganisms in fish-rice ecosystem in soda saline-alkaline land of the Songnen Plain.The results showed that population of heterotrophic bacteria in the fish-rice （ecosystem） was 2 800—307 160 mL^-1,higher than that in the normal rice farming system（P<0.05）,with the order of rice growth last-term>middle-term>primary-term.The density of actinomyces and molds was low in both the two ecosystems.The population of coliform in the fish-rice （ecosystem） was 403.6—17 934.2 L^-1,higher than that in the normal rice farming system（P<0.01）,with the order of rice growth middle-term>primary-term>last-term.The population of heterotrophic bacteria was significantly related to fish production,total amount of fertilizer applied and quantity of organic manure applied,but not to the application rate of chemical fertilizer;the population of coliform was also closely related to fish production,but not to the total amount of fertilizer applied.Nevertheless,it showed extremely significant relationship with the application rate of organic manure.Ten genera of heterotrophic bacteria were detected in the fish-rice ecosystem with Vibrion,Aeromonas,and Bacillus being the dominant groups.