A protocol to quantify ustiloxin A (ustA) in a bulk sample of false smut balls of rice (J. Vet. Med. Sci. 71: 239-241) was modified to quantify ustA in single balls collected from a paddy field after natural infection and in balls formed after rice plants were inoculated with either of two isolates of Ustilaginoidea virens isolated from different commercial fields in Fukui Prefecture. To test whether these isolates differed in their capacity to produce ustA, we compared the mean quantity of ustA that each produced in individual balls. The two isolates varied greatly in ustA production; the differences in ustA in balls between the two ranged from 42-fold to 115-fold. Such massive differences in ustA production indicate that the pathogenic isolate is an important contributing factor to the quantity of ustA produced in false smut balls.