Abstract The populations of chemolithoautotrophic (colorless) sulfur bacteria and anoxygenic phototrophic bacteria were enumerated in a marine microbial mat. The highest population densities were found in the 0–5 mm layer of the mat: 2.0 × 10 9 cells cm −3 sediment, and 4.0 × 10 7 cells cm −3 sediment for the colorless sulfur bacteria and phototrophs, respectively. Kinetic parameters for thiosulfate-limited growth were assessed for Thiobacillus thioparus T5 and Thiocapsa roseopersicina M1, both isolated from microbial mats. For Thiobacillus T5, growing at a constant oxygen concentration of 43 μmol l −1, μ max was 0.336 h −1 and K s 0.8 μmol l −1. Phototrophically grown Thiocapsa strain M1 displayed a μ max of 0.080 h −1 and a K s of 8 μmol l −1 when anoxically grown under thiosulfate limitation. In a competition experiment with thiosulfate as electron donor, Thiocapsa became dominant during a 10-h oxic/14-h anoxic regimen at continuous illumination, despite the higher affinity for thiosulfate of Thiobacillus.