Bacterial production of cytokinin-type and auxin-type plant hormones was investigated with 328 strains isolated from seawater, sediment and various alga samples. Proportions of the producers among heterotrophic marine bacteria were quite high, 10-80%, showing higher rate in the samples from multicellular algae and sediments than those from unicellular algae and seawater. The amount of auxin, indole-3-acetic acid, produced in the culture was mostly in the level of 10^-7mol/l, which was about 100 fold greater than that of cytokinin compounds. Marine Gram-negative bacteria capable of the cytokinin and the auxin production were classified in 9 groups by a combination of key taxonomic characters. In the alga samples, there were some differences in taxonomic groups between cytokinin- and auxin-producers, but as a whole the ability of plant hormone production tended to distribute in various genera of marine bacteria.