Acinetobacter cakoaceticus P23 is a plant growth-promoting bacterium that was isolated from the surface of duckweed (Lemna aoubilmsa). The bacterium was observed to colonize on the plant surfaces and increase the chlorophyll content of not only the monocotyledon Lemna minor but also the dicotyledon Lactuca sativa in a hydroponic culture. This effect on the Lactuca sativa was significant in nutrient-poor ( x 1/100 dilution of H2 medium) and not nutrient-rich ( x 1 or x1/l0 dilutions of H2 medium) conditions. Strain P23 has the potential to play a part in the future development of fertilizers and energy-saving hydroponic agricultural technologies. (C) 2013, The Society for Biotechnology, Japan. All rights reserved.