The final microflora in the corrosion process of concrete sewer pipes was investigated. When the corroded sample was examined using several media, bacterial colonies were found only on acid media (pH 2.5); fungi were detected on neutral solid media (pH 6.5) as well as on acid media (pH 2.5). The acidophilic bacterial colonies were identified as Thiobacillus thiooxidans using a specific identification method for species of acidophilic thiobacilli. The dark green fungi that appeared on the isolation media showed similar morphological characteristics, even though the media used for isolation varied in pH and nutrient. The fungi showed tolerance against acid, although the optimum pH for their growth was neutral. The results showed that the severely corroded sewer pipe was inhabited by two kinds of microorganisms, Thiobacillus thiooxidans and the fungi. An isolated fungus, strain OMSOfl, could oxidize sulfide to thiosulfate. Thiosulfate can be utilized by T. thiooxidans as an energy source, and is converted to corrosive sulfate. Continued vigorous growth of T. thiooxidans presumably depends on a mutualistic relationship with the fungus. It is proposed that a close association between the two microorganisms accelerates the corrosion of concrete sewer pipes.