Biogenic H2SO4 corrosion of concrete surfaces caused by thiobacilli was reproduced in simulation expts. At 9 mo after inoculation with thiobacilli, concrete blocks were severely corroded. The S compds. H2S, S2O32-, and MeSH were tested for their corrosive action. With H2S, severe corrosion was noted. The flora was dominated by Thiobacillus thioxidans. S2O32- led to medium corrosion and a dominance of Thiobacillus neapolitanus and Thiobacillus intermedium. MeSH resulted in negligible corrosion. A flora of heterotrophs and fungi grew on the blocks. This result implies that MeSH cannot be degraded by thiobacilli.