Imposer was measured in the rock shell, Thais clavigera, from the coast of the Korean peninsula. Frequency of imposer was 0% at two reference sites, but at 47 out of 61 sites, frequency of imposer was recorded as 100%. The degree of imposer was relatively high at the sites near a harbor and a shipyard. Tributyltin (TBT) and triphenyltin (TPT) concentrations in T. clavigera ranged from 5 to 508 ng/g and from 3 to 2460 ng/g, respectively. A significant positive relationship was found between degree of imposer and organotin concentration, whereas a significant negative relationship was obtained between female-to-male sex ratio and the degree of imposer. During a field transplantation of T. clavigera from a pristine area to a port, TBT and TPT were accumulated in T. clavigera, and imposer was induced. T. clavigera shows considerable potential as a bioindicator species of the adverse effects of TBT and TPT contamination. (C) 2000 Elsevier Science Ltd. All rights reserved.