Sixteen biocides were examd. for their ability to inhibit the growth of thiobacilli. Biocide A inhibited the growth of Thiobacillus thiooxidans, and biocide B the growth of T. neapolitanus, T. intermedius, and T. thiooxxidans. Biocides A and B were effective at concns. =10 times higher. Concrete blocks were treated with 2 concns. of A and B (10 and 30 g/cm2, resp.). The blocks were incubated in a H2S test chamber for the simulation of biogenic H2SO4 corrosion. During the 10-mo test period for nos. of thiobacilli on the surface of the treated concrete blocks, with the higher concn. of biocide, remained 10-102 below the cell counts of the untreated control block and those treated with the lower concn. The wt. loss amounted to 2%, whereas the control lost 6%.