Blast disease, caused by Pyricularia grisea, is one of the most serious diseases of rice in Thailand. To study the diversity in this fungus, rice samples showing blast symptoms on the leaf, neck, collar, panicle, and seed were collected from rice fields in various locations in Thailand. During the collection, it was observed that the popular cultivars grown by farmers, KDML105 and sticky rice, had high disease severity, especially in northern and northeastern Thailand. Several cultivars, such as Hom Klong Laung and Hom Suphan, were highly resistant in some locations byt susceptible in others. In addition to differences in cultural characteristics of Pyricularia grisea between isolates, we found high diversity in spore morphology among isolates from different locations. Moreover, results of the pathotype test indicated that many isolates were compatible with different cultivars. When examined for mating type, results showed 123 blast isolates of type I and 21 of type II.