Phenetic data on over 60 heterotrophic, Gram-negative, yellow chromogenic bacteria from plant material were collected and analysed using numerical taxonomic methods. Marker strains representing 42 taxa were included in the analyses. At similarity levels of 80% or above, eight distinct clusters were obtained, the first four of which included yellow chromogens. Custer I contained isolates from green healthy leaves of Agrostis tenuis, Festuca rubra, Holcus lanata, Lolium perenne and Poa pratensis, and clusters 2 and 3 consisted of isolates from Holcus lanata seeds and leaves of P. pratensis respectively. Cluster 4 contained seven subgroups and was equated with the family Enterobacteriaceae. Erwinia herbicola strains from a variety of sources formed a homogeneous subgroup, readily distinguishable from authentic strains of E. amylovora, E. carotovora, other representative erwiniae, and from all other enterobacteria studied. These data emphasize the heterogeneous nature of yellow-pigmented bacteria from plants, and support the inclusion of E. herbicola and other Erwinia species in the Enterobacteriaceae.