蚵岩螺（Thais clavigera）常見於台灣西部沿海的牡蠣養殖區，喜食牡蠣，被蚵農視為有害動物。受環境污染時，雌螺易長出雄性生殖構造，雄性生殖構造一旦產生便無法逆轉，此狀況稱之為性變異（Imposex）。本研究自2013年2月起至隔年2月止，分別於雲林縣台西與台南市北門的牡蠣養殖區進行採樣，並以性變異率（Incidence of Imposex, IOI）及陰莖相對大小指數（Relative Penis Size Index, RPSI）對蚵岩螺進行性變異程度分析。台西的蚵岩螺平均殼長為37.5± 6.3 mm，平均濕濕重為1.5± 0.8 g（N= 473）；北門蚵岩螺平均體長為39.8± 7.9 mm，平均濕濕重為1.9± 1.3 g （N= 378）；雌雄性別比皆大於2。兩樣區的雄性蚵岩螺在生殖季節（1~4月），其陰莖明顯變長變粗，但有性變異的雌性蚵岩螺陰莖則變化不明顯。台西蚵岩螺的I0I值為20%，北門則為51%；就發生性變異機率而言，北門蚵岩螺發生的機率高過於台西。台西蚵岩螺的RPSI值為11%，北門則為2%；就性變異的程度而言，台西樣區約為北門樣區的5.5倍；台西樣區的蚵岩螺在性變異程度上明顯大於北門樣區。 Thais clavigera is a common sea snail in the oyster farming areas of Taiwan's west coast. Living on this aquatic animal, Thais clavigera is regarded as a pest by oyster farmers. When lavigera is affected by environmental pollution, the female tends to develop a male sex organ, which cannot be reversed once formed. This disorder is named "imposex". Sampling work for this study was conducted from February 2013 to February 2014 in the oyster farming areas at Taishi, Yunlin and Beimen, Tainan. The analyses of imposex degree among Thais clavigera individuals were performed by Incidence of Imposex (IOI) and Relative Penis Size Index (RPSI). The colony of Thais clavigera at Taishi site had 37.5± 6.3 mm in length, and 1.5± 0.8 g in weight (N= 473); the colony at Beimen site had 39.8± 7.9 mm in length , and 1.9± 1.3 g in weight (N= 378); the female-to-male ratios at the two sites were both greater than 2. The penises of the male Thais clavigera at both sites became noticeably longer and thicker during reproductive season (January to April); however, those of the imposex-exhibiting female Thais clavigera showed no obvious changes. The Thais clavigera colony at Taishi site had an IOI value of 20%; 51% at Beimen site. In other words, the colony at Beimen site had a higher chance of imposex than that at Taishi site. The Thais clavigera colony at Taishi site had an RPSI value of 11%; 2% at Beimen site. In terms of imposex degree, the RPSI value at Taishi site was 5.5 times higher than that at Beimen site. We can see a significant difference between these two sampling sites.