In order to analyse oxidized sulfur compounds in air, determination of sulfate in rain water is important. For this purpose, it is necessary to analyse many samples containing a small amount of sulfate. Previously, a microbiological assay of sulfate in water using cultivation of Escherichia coli B was developed by Omura. We applied Omura's method on our object and defermined sulfate in rain water using Escherichia coli B (H). The standard curve was established by measuring turbidity of the culture medium, and showed lineality in the range from 0 to 12 ppm of sulfate. The amount of 0.5 to 20 ppm of sulfate in rain water was determined by this method with an accuracy of standard deviatio n<±3%.