The aim of this study is to obtain knowledge about which bacterial species are present in indoor air in homes, and whether physical factors affect the concentration and species composition of bacteria in indoor air. Measurements have been performed for one whole year inside five homes and once in 54 homes. The most species rich genera were Bacillus, Paenibacillus, and Kocuria. Among the gram-negative bacteria Paracoccus yeei was found frequently, and the concentration of this bacterium was associated significantly with season. The concentrations of gram-positive and gram-negative bacteria and species compositions were significantly associated with season. The concentrations of Kocuria palustris, Kocuria rhizophila, and Bacillus pumilus were associated positively or negatively with either absolute or relative humidity.