Fluoroacetate was extremely toxic toThiobacillus neapolitanus strainC, retarding growth even at 10-9m. Inhibition could be relieved by relatively high concentrations of acetate or propionate. Inhibited cultures eventually recovered from inhibition and grew in the presence of fluoroacetate over the concentration range 10-9 to 10-5m. The recovery represented a recommencement of division of the total population, as it was shown that 60–100% of the organisms inoculated on to agar containing as much as 10-3m fluoroacetate formed colonies after lags as long as 37 days. Even longer lags occurred with more fluoroacetate, but fewer organisms survived. Fluoroacetate appeared specifically to inhibit the Krebs' cycle through fluorocitrate synthesis; this confirmed that the cycle is essential to the autotrophic metabolism.