Corrosion of concrete in sewer pipes is the result of the activities of thiobacilli, which are found in the greatest nos. in large, corroded holes and are responsible for biogenic H2SO4 formation. Corrosion proceeds slowly when the concrete is protected by a closed, thick layer of slime. Under these conditions, there are relatively few thiobalcilli and they have almost no activity. Since H2S was found in traces at most, it is not clear what compds. are being oxidized to H2SO4. Thiobacillus thiooxidans Cannot live under these conditions; the characteristics of the responsible thiobacilli are discussed.