Enhancement of elongation ofAvenaapical coleoptile segments byDL-fluorophenylalanines is specific for the para isomer. The ortho isomer is less than one-third as effective and the meta isomer has no effect. The in vitro activity of extractable phenylalanine ammonia lyase is inhibited equally well by para- and ortho-fluorophenylalanine and more strongly by the meta isomer. Inhibition was of the competitive type in all three cases. Chlorogenic acid levels were used to indicate the inhibition of phenylalanine ammonia lyase activity in vivo. Chlorogenic acid declined in controls. This decline was accelerated by the para isomer but not by the ortho isomer. The meta isomer protected against loss of chlorogenic acid. Enhancement of elongation was partially blocked bytrans-cinnamic acid, ferulic acid and coumarin but not by chlorogenic, caffeic, oro-coumaric acids. It is postulated thatp-fluorophenylalanine enhances elongation through depressed deamination of phenylalanine and consequent lowering of potentially inhibitory low molecular weight phenolic constituents.