Tributyltin (TBT) is widely used as a biocide in antifouling paints. Astyanax bimaculatus adult fish were acclimatized in a laboratory and isolated in groups of eight individuals. Two groups were used as a control (similar handling and corn oil control) and one group was exposed to TBTCI dissolved in corn oil (0.0688 +/- 0.0031 microg TBT.g(-1)) every 6 days for 32 days. Light and transmission electron microscopy were used to investigate the morphological effects on liver. A muscle fragment was excised for the determination of the acetylcholinesterase activity and blood smears were obtained for differential white cell counts. The results indicated nuclear irregular shapes, chromatin condensation, presence of intranuclear lipid bodies, and degenerative nuclei. Acetylcholinesterase activity was not affected by TBT exposure. The increasing number of neutrophils may represent cytotoxic and stress conditions facilitating the invasion of opportunist organisms. According to the authors' data, TBT was confirmed as a powerful aquatic contaminant to fish in tropical ecosystem.