Several strains ofPseudomonas mors‐prunorum(Wormald) andPs. prunicola(Wormald) isolated from pathological lesions of plum and cherry were studied together with the causal organism of bacterial canker of stone‐fruits in California(Ps. syringaefrom apricot) and other phytopathogenic bacteria obtained from pear and syringa. Comparison was also made with pseudomonas forms pathogenic to pea, bean, lettuce, and tobacco, and with the common saprophytesPs. fluorescens and Ps. pyocyaneus.With the exception of two yellow organisms(B. pruniand the Pear 8 strain—the latter, however, very occasionally showing fluorescence), all belong to the green‐fluorescent group ofPseudomonas(Dowson's Group II). On the basis of their dissimilation of C and N compounds a very close relationship has been established between these fruit‐tree and syringa pathogens of the green‐fluorescent group.Ps. mors‐prunorumis not highly specialized in its nutrient requirements but can satisfy its fundamental C and N requirements from a very large variety of simple substances. The only consistent biochemical differentiation shown byPs. mors‐prunorum(including some of the syringa strains) in comparison withPs. prunicola(includingPs. syringaefrom apricot and most of the pear strains) is its more rapid production of add from sucrose. Both themors‐prunorumandprunicolavarieties produce a levan from sucrose, which causes a raised gummy growth on solid sucrose‐containing media. This applies also toPs. pisi, Ps. tabaci, and Ps. phaseolicola, but is not the case with the weakly pathogenic forms—Ps. marginalis, cerasi (= trifoliorum, from bean), and the saprophytes—Ps. fluorescens and Ps. pyocyaneus.On the basis of biochemical characteristics, considered apart from host pathogenicity, there is no justification for erecting to specific rank these various levan‐forming. green‐fluorescent, p