The guppy （Poecilia reticulata）is small freshwater fish native to South America from Venezuela to Brazil and has been introduced in Japan not only as ornamental fish but also as a biological control to mosquito. These introductions resulted some of populations colonized as an exotic species in Okinawa southern Island of Japan. The present study aimed to estimate introduction sources of the guppies in Japan by means of sequence analysis of 320 bp of mtDNA D-loop region. Sixteen wild populations and fourteen ornamental strains were examined. Six haplotypes (Hap1 ~ Hap6) were detected from the sequence analysis. All ornamental strains except Endrer's and populations collected in Singapore, Thailand and Venezuela had Hap1 which suggest the introduction sources of the haplotype from West Trinidad. Endrer's has Hap4 which suggest the introduction from East Venezuela and this haplotype was observed only in Endrer's in the present study. On the other hand, colonized populations collected from Okinawa Island have five types of haplotype, Hap1, Hap2, Hap3, Hap5 and Hap6. These haplotypes were originated from wide area of Northern South America. Hap2 was observed in North Guyana, Hap3 was observed in East Trinidad, Hap5 was observed in Suriname and East Guyana and Hap6 was observed in West Trinidad. Genetic variabilities in colonized populations of Okinawa Island were lower than that of original populations in South America. These results suggest that the origin of ornamental strain in Japan was originated from specific narrow area and small number of individuals, probably from West Trinidad. Meanwhile data of colonized populations in Okinawa Island suggested occurrence of multiple introduction or invasion from many origins from different area.

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