The genetic structure of oyster drill Thais clavigera along the west coast of Taiwan were assayed by starch gel electrophoresis. Factors of locality¡]i.e. Shainsan, Taisi, Budai, Chiku¡^,maturity¡]i.e. mature, immature¡^ and sampling time¡]i.e.1999.7~2000.3, 2000.11¡^ were analyzed to evaluate their effect on drill¡¥s genetic structure . Four of the eleven investigated enzyme loci were polymorphic , i.e. Ark, Lap-1, Lap-2, and Pgm-1. Among the four populations , the mean heterozygosity¡]H¡^and genetic distances¡]D¡^ ranged from 0.100 to 0.129 and from 0.0005 to 0.0029, respectively. Therefore, T. clavigera along the west coast of Taiwan belongs to the same population. However, differentiation among populations still existed which was contributed by Ark locus. According to the degree of differentiation, the four populations were ranked as Shainsan¡BBudai¡BTaisi and Chiku . A similar trend was also observed in the four populations, as the drill size increased, the mean heterozygosity and the heterozygosity of Ark locus increased. Sampling time had no significant effect on genetic structure of populations. It is suggested that environmental factors¡]e.g. environmental quality and topography¡^and drill¡¦s reproductive ecology may be important in determining the genetic structure of populations.