Abstract: Community structure and physiological characteristics of heterotropic pacteria at three stations in the Donghu Lake and a deep well water were studied. Bacteria were isolated randomly on nutrient agar, and each strain was characterized in morphological, physiological properties and substrate utilization capabilities. Based on 113 features for each strain and their similarity degrees, cluster analysis and diversity index (Shannon-Weaver index) were used for describing structure and heterogeneity of the communities. The physiological activity levels and the substrate utilization patterns of the four heterotrophic bacterial communities were compared. The results indicated that the communi ies in the lake water differ from those in the ground water in several ways: 1. The lake water and the ground water communities differed in their quatitative and quantitative morphotype composition. Gram-negative, non-spered and coloured strains were predominant groups in the ground water; more Gram-positive bacteria and the strains were observed in the lake water with fewer pigments. 2. Diversity index value of the heterotrophic bacterial community was ranked in the order Donghu Lake Station 1>Station Ⅱ>Statio Ⅲ>ground water community. 3. The total activity of strains and single and total activity of community were lower in ground water than in lake water; in the lake thre was no significant difference among three stations. 4. The substrate utilization patterns were different between the lake and the ground water communities. But the three communities in the)onghu Lake were hardly distinguishable from each other in the pattern of substrate utilization.