Xanthate floatation compounds, added to cultures of Thiobacillus ferrooxidans leaching chalcopyrite, inhibited growth in the concentration range 1-10 mM and caused a drop in formation of soluble copper and iron. Maximum(80-90%) growth inhibition was at 10 mM for isobutyl-, amyl- and ethyl xanthate while isopropyl xanthate was innocuous but caused a four-fold increase in lag phase. Copper production was depressed by 30% for isopropyl-, 53% for isobutyl-, ethyl- and 77% for amyl xanthate at 10 mM additions.