A unique microbial community exists on upper leaf surfaces because strong irradiation by the sun restricts and selects growing micro-organisms. The micro-organisms on plant aerial parts are possibly able to control plant pathogens and to promote plant growth as do some of the micro-organisms on plant roots. Sphingomonas species on plant tissues (seed, leaf and flower) were detected and enumerated by the amplification of a specific part of the small subunit rRNA gene. Sphingomonads were found on 26 plant species belonging to 11 families. The maximum population number of Sphingomonas was 108 g−1 (wet weight) plant tissue. The maximum population ratio of Sphingomonas to the total aerobic bacteria was 58% when it was counted on glucose-nutrient agar plates. Consistent differences in population number and population ratio were not found among seeds, leaves and flowers. The role of sphingomonads on plant aerial parts is discussed.