As a consequence of the widespread industrial and agricultural applications of organotins, contamination of various ecosystems has occurred in recent decades. Understanding how these compounds interact with microorganisms is important in assessing the risks of organotin pollution. The organotins, tributyltin (TBT), trimethyltin (TMT) and inorganic tin, Sn(IV), were investigated for their physical interactions with non-metabolising cells and protoplasts of the yeast Candida maltosa, an organism that is often associated with contaminated environments. Uptake, toxicity and membrane-acting effects of these compounds, at concentrations approximating those found in polluted environments, were assessed. Sn(IV) and TBT uptake occurred by different mechanisms. Uptake of Sn(IV) was 2-fold greater in intact cells than protoplasts, underlining the importance of cell wall binding, whereas TBT uptake levels by both cell types were similar. TBT uptake resulted in cell death and extensive K+ leakage, while Sn(IV) uptake had no effect. TMT did not interact with cells. Of the three compounds, TBT alone altered membrane fluidity, as measured by the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene incorporated into cells. Anisotropy of 1-(4-trimethylaminophenyl-6-phenyl-1,3,5-hexatriene) was not affected, implying that TBT is not confined to the surface of the cytoplasmic membrane, but acts within membrane lipids. These results indicate that the cell wall is the dominant site of Sn(IV) interactions with yeast, while lipophilic interactions play an important role in uptake and toxicity of TBT.