1. Activities of proteinase, urease, sucrase and dehydrogenase in red soils increased with the improvement in soil fertility. There was a positive correlation between the enzyme activity and the respiratory capacity. The study of the microbiological characteristics of red soils showed clearly that the abundance of soil microorganisms, respiratory capacity and enzyme activities are correlated with the improvement in soil fertility, and as a consequence such a correlation may be used as an index of soil fertility. 2. The activities of proteinase, urease and sucrase decreased with the increase of soil depth in fertile soils, but such a tendency was not so clear with dchydrogenase and peroxidase activities as with the above mentioned enrymes.