A newly discovered bacterium, Desulfotomaculum auripigmentum, precipitates arsenic trisulfide (As2S3). Precipitation of As2S3 by this organism results from its reduction of As(V) to As(III) and S(VI) to S(2II). At the As(III) concentration range of interest (0.1 to 1 mM), the stability of As2S3 is highly sensitive to pH and [S(2II)]. Thus, the relative rates at which D. auripigmentum reduces As(V) and S(VI) are critical to its formation of As2S3. Other As(V)- or S(VI)-reducing bacteria are unable to precipitate As2S3 either due to their inability to reduce both As(V) and S(VI) or because they reduce S(VI) too rapidly. Electron microscopy of thin sections showed that the precipitate forms both intra- and extracellularly. Microbial As2S3 formation nucleates precipitation of the mineral in the bulk milieu, whereas heat-killed cells alone do not serve as templates for its formation. Precipitation of As2S3 by D. auripigmentum suggests that As2S3 formation may be important in the biogeochemical cycle of arsenic.