Imposex syndrome is the superimposition of a pseudopenis and vas deferens in the reproductive system of female gastropods. This anomalous virilization is attributed to a hormonal imbalance induced by tributyltin (TBT), an organotin compound, present in antifouling paints. This study analyzes the severity, extent, degree and indices of imposex in a population of Thaisella chocolata from Caldera Bay, Atacama Region, Chile. Sex was determined by biopsy of the gonad from 180 specimens and imposex by the concurrence of ovarian, capsule gland, vas deferens and pseudopenis in abnormal females. The proportion of females affected with imposex was estimated using the formula described by Gibbs et al. (1987). The degree of the syndrome progress was classified using five scales of imposex and Vas Deferens Sequence Index (VDSI), Relative Penis Index (RPSI) and Relative Penis Size Index (RPSI) were calculated. Results show that 100% of the females of T. chocolata from Caldera Bay suffer imposex. Macroscopic analysis revealed atrophy of the vagina and femenine glands. Some females lacked ingestive gland and seminal receptacle. Cytological disorders as modification of internal epithelium, occurrence of acellular areas, lack of basal lamina and presence of pyknotic nuclei, and overlapping of prostate tissue in the three females’ glands were found. Anatomical and histological alterations associated with imposex determine the absolute sterility of female. The syndrome reached maximum degrees in all scales except for that of Barreiro et al. (2001). It is recommended the use of VDSI for T. chocolata, since provides a realistic measure of the virilization and reproductive potential of the females and would not be affected by the shape and size of the penis.