Isozymes of 5 wild-caught collections of Aedes aegypti were compared by using starch gel electrophoresis. All collections were collected from Surat-Thanee Province, southern Thailand, an area considered to be a hyperendemic zone for dengue virus. One collection was from Donsak Harbor, whereas the other 4 collections were from 4 districts of Samui Island. The percent polymorphic loci (24.2-33%) in the 4 island collections was lower than in the mainland collection (36.4%). This study revealed a large effective migration rate among all 5 collections. No fixed differences were detected. No significant differentiation was found among the 5 collections from Surat-Thanee Province.