Plant pathogenic pseudomonads such as Pseudomonas glumae, Ps. gladioli pv. gladioli, Ps. cepacia and Ps. avenae formed growth inhibition zone around their colonies on the lawn of other plant pathogenic bacteria, Agrobaclerium tumefaciens, Corynebacterium michiganense pv. michiganense, Erwinia carotovora subsp. carotovora, Ps. solanacearum, Ps. syringae pv. syringae, Xanthomonas campestris pv. citri and X. campestris pv. oryzae, used as indicators. The activity spectra shown on the indicator bacteria were much varied depending upon the species and isolates of pseudomonads, suggesting that various antibiotic substances participated in the formation of inhibition zones.