Genetic relatedness among 51 strains of fluorescent pseudomonads comprising 18 plant pathogenic species and two saprophytic species were determined by in vitro nucleic acid hybridization. Four of six homologous groups, shown to be distinct genotypically, are provisionally designated as: “syringae,” “morsprunorum,” “viridiflava,” and “cichorii.” Two other homology groups seemed to be distinct, but because of lack of tests with reference strains the amount of intragroup relatedness is unknown. Thus, these somewhat artificial groups are provisionally designated “tomato” and “marginalis.”