Red soils derived from quaternary red clay occur on the hilly land of Chuhsien, western Chekiang. The soils have been cultivated for a long time, and their fertility was definitely improved as the cultivation went on. Four samples of these soils of different fertility, varying in accordance with their history of cultivation, were used in this study, and the microbial population, rate of nitrification and cellulose decomposition, and the respiratory capacity of these soils were analysed 4 times annually. The results were summarized as following: 1. The abundance of soil microorganisms increased with the improvement of soil fertility. Among the microbial population, bacteria is the most abundant organism in all soils. The number of bacteria is relatively higher in more fertile soil, while the relative number of fungi is decreased as the cultivation went on. The abundance of aerobic cellulose decomposing organisms, the nitrifying bacteria and aerobic nitrogen fixing bacteria are also positively correlated with soil fertility.