Homoserine inhibited fluorescent phytopathogenic pseudomonads of the Pseudomonas syringae group. An intermediate in a branched pathway of amino acid biosynthesis, homoserine was not toxic in the presence of the pathway's end products, methionine, threonine and lysine. Various inhibitions in four branched biosynthetic pathways by certain amino acid end products, in the absence of others, lead to cessations of growth of phytopathogenic strains representing P. coronafaciens var. atropurpurea, P. glycinae, P. helianthi, P. mori, P. phaseolicola, P. syringae and P. tabaci. Saprophytic strains representing P. fluorescens and P. putida were only inhibited by cysteine, an inhibitor of all the strains studied. Conditions of free amino acid imbalances, as exist in plants, may lead to inhibitions of these pathogens and may in some instances determine their range of compatible hosts and tissues.