The amounts of organotin compounds such as butyltins [sigma BTs: tributyl- (TBT), dibutyl- (DBT), monobutyl- (MBT)] and phenyltins [sigma PhTs: triphenyl- (TPhT), diphenyl- (DPhT) and monophenyl- (MPhT)] in rock shells, Thais clavigera, from the northern (Shiangsan), central (Lukang) and southern (Chiku) Taiwan oyster mariculture areas varied with season and location. High values of sigma BTs (mainly TBT) and low values of sigma PhTs (mainly TPhT) were found during winter (January 1999) at Shiangsan, while high amounts of sigma PhTs (mainly TPhT) and low sigma BTs (mainly TBT) were found during summer (August 1998) at Lukang. The snails were also affected by imposex. Imposex at Shiangsan, Lukang and Chiku increased from 67.1, 59.3 and 36.7% in summer to 100, 100 and 80% in winter, respectively. Relative penis size indices similar trend at Chiku site. No male displayed rock shells were found at Shiangsan and Lukang in winter. Linear correlations between pseudopenis length versus TBT (r = 0.7655, P < 0.001), DBT (r = 0.4253, P < 0.05), MBT (r = 0.5865, P < 0.01) and TPhT (r = -0.6160, P < 0.01) were obtained significant. Among 200 samples, significant positive correlations between length (pseudopenis/shell) ratio of female versus TBT (r = 0.6944, P < 0.005) and sigma BTs (r = 0.6413, r < 0.01) were also observed. The weak correlations between length (pseudopenis/shell) ratio of female versus DBT (r = 0.3085) and MBT (r = 0.4240, P < 0.1) and TPhT (-0.4917, P < 0.05; negative correlation) could indicate that DBT, MBT and TPhT have little or no effect on the development of imposex in rock shells.