Influence of nitrogen and sulfate concentrations in wastewater on a purple non-sulfur bacteria (PnSB) pond process were investigated. A laboratory-scale chemostat photobioreactor treating four types of simulated wastewater were tested with different combinations of nitrogen and sulfate concentrations. Rhodopseudomonas palustris and sulfate-reducing bacteria were detected by a fluorescent in-situ hybridization (FISH) technique using specific probes, and their population was quantified by pixel counting in fluorescent microscopic observations. As a result, ratios of Rpal686 on EUB338 probe were over 25% throughout the operation period. Average removal of dissolved organic carbon was 94% independent of nitrogen and sulfate concentrations. However, it was found to be difficult to suppress sulfate reduction, even in case that nitrogen-limited wastewater was fed.