Bacillus species were observed and quantified by molecular approaches, using the 16S rDNA primers/probes, in a wastewater treatment plant designed for the purpose of stimulating the growth of Bacillus species. The plant has been operating as a test plant since 1997 in the city of Ina, Japan, with excellent treatment performance. Observations by in situ hybridization, using Bacillus-specific probes, indicated that Bacillus strains were inhabited in the plant and their num­bers decreased during the treatment process. Similar results were obtained from a quantitative PCR analysis using a Bacillus-specific primer set, and the amount of DNA originating from various Bacillus species was maximally $1.91%$ of the total DNA in the wastewater treatment tank. Clone library analysis using the Bacillus-specific primers suggested that, while the population was no­ticeably increased, the phylogenetic diversity of the increasing Bacillus species was very low.