In Japan, bathwaters are a potential source of legionellosis; therefore, the bathwater in public facilities must be regularly tested for Legionella spp. Currently, Legionella spp. is detected using the culture method, which is time-consuming and has limited accuracy. Moreover, the reproductive environment of Legionella spp. in bathwater remains unclear. Here, we investigated the environment in which Legionella spp. reproduce by analyzing the bathwater microbiota and its relationship with Legionella spp.