The present paper studies the populations of Sagittaria pygmaea Miq. in Hubei province. The results obtained are as follows: 1. Almost every classificatory character has variation in varying degrees. The range of variation is wider than that of records in literatures, and presents each regularity. 2. The various abnormal individuales discovered by us, can provide some important leads for us to study the evolution of S. pygmaea. 3. The length of scapes in every population is closely related to the depth of water. 4. the variable range of stamens in number is from 6 to 21 each male flower. Basically, it reduces in 3-stamens. 5. Two important advance trends, male plant differentiation and a decrease in stamens, there is a correlation between them.