Intraspecific heterogeneity among 27 Asian monoconidial isolates and four ATCC ex-type cultures of Monascus purpureus from red rice and sufu was revealed as mycelial compatibility groups (MCGs) using mycelial reactions of 496 mycelial parings. Among 88 pairs, at least three kinds of mycelial incompatible reactions were found at the interaction zone of confronting colonies as: (1) closely forming but not overlapping colonies; (2) sparsely forming aerial mycelia; and (3) thick mycelial band followed by mycelial deterioration of one partner. A mycelial-free zone was not found in any interaction. Compatible interactions were evidenced by freely intermingled hyphae with no apparent inhibition of growth of either partner. At least six MCGs were found based on compatible reactions. MCG1 and MCG2 consisted of 11 isolates, mostly from Thailand. Also, MCG1 contained two ATCC ex-type cultures, ATCC16365 (from Indonesia) and ATCC26264 (from Taiwan). In addition to nine Thai isolates, MCG2 included T9·1 isolate from China and isolate ATCC16427. Furthermore, four small MCGs of M. purpureus were identified. MCG3 (isolates A4·8 & TT4·2) and MCG4 (isolates 10 & M6) were comprised of isolates from Thailand. MCG6 consisted of Japanese isolate M22, and ATCC16360. Isolate ATCC16360 was of unknown origin and previously had been identified as M. anka. MCG5 consisted of Japanese isolate M20 and two Thai isolates, M13 & M14. The result indicated the Thai isolates of MCG1 (16·1, T8·6, TT2·3, TT5·1, M1, M2, M3, M7 & SR) were the same clone as those isolates from Indonesia and Taiwan. Also, the Thai isolates of MCG2 (4, 8, 11, TT1·1, M4, M8, M15, M17 & M18) along with the Chinese isolate T9·1 and ATCC16427 were from the same genetic origin. This study was correlated to those of RAPD analysis of genetic variation within a collection of Monascus spp. isolated from red rice and sufu. M. purpureus isolates used as a starter culture of red rice production among Asian countries were from a rather restricted genetic pool, e.g. China, Taiwan and Japan, although the isolates of M. purpureus studied were moderately genetically diverse.