Abstract: >We studied the community structure and diversity of cultivable heterotrophic bacteria in West Lake, Hangzhou from October 2001 to October 2002. Results showed that there were 510 CFU of bacteria per mL water and 2.27×10⁵ CFU per gram of dry sediment. The bacterial population in water was higher in cold seasons than in hot seasons. In sediments, the bacterial population varied not only with seasons but also with dredging of the mud. Soon after dredging of mud in Changqiao,bacterial counts were about 0.81×10⁵ CFU per gram in the new sediment. After three months, the bacterial population had increased to 4.60×10⁵ CFU per gram. There were 14 types of cultivable heterotrophic bacteria in the West Lake ecosystem. The dominant types were Pseudomonas,Enterobacteriaceae or Aeromonas, which accounted for 36.8%~80.4% of the total bacterial population in water column; and 27.7%~72.7% of those in sediments. Analysis of Shannon-Wiener index (H) and Simpson index (L) indicated that the Lake had low bacterial diversity. The index H was between 0.682 and 1.787 in water columns; and between 0.809 and 1.774 in sediments, and the index L was between 0.196 and 0.662 in the water column, and between 0.179 and 0.572 in sediments. Among the bacteria from water column, 95% were Gram negative rods, and 20.1% produced yellow pigments. In sediments, about 85% of the bacteria were Gram negative rods and only 2.07% produced yellow pigments. Furthermore, the population of Bacillus was much higher in sediments than in the water column, suggesting that there are different niches in water column and sediments in West Lake despite the frequency of water-mud exchange.