One thousand forty-four isolates of Pyricularia oryzae obtained during the period 1962-1974 front blast-infected rice samples from all rice-growing areas of Korea were tested on the Japanese differential set of twelve rice varieties. Twenty-seven races of the fungus were differentiated. The number of races falling in groups T,C and N were 4, 15 and 8, respetively. Since only Japonica type varieties had been cultivated widely before 1966, N-race groups were found to be most Prevalent with a frequency of about $85\%$ of the total isolates. After 1966, T-and C-race groups gradually increased to $21\%$ and $42\%$ respectively, as a result of the widespread cultivation of Tongil bred from a cross between IR8 x(Yukara x T(N)1. Before 1966, a few T, and C-race groups occurred only in the Suweon region; since 1966, most of the races differentiated have been uniformly distributed throughout the country regardless of region. The most prevalent races in Korea were N-2 and C-8. The variety Tetep was resistant to all races previously identified in Korea, but a new race (T-d) to which Tetep is susceptible has been found each year since 1973. It appears likely that races capable of attacking Indica types of rice will become increasingly prevalent and a programme of continuous screening is needed to detect new races as soon as they appear.