We have isolated three classes of cDNAs that probably encode three 4-coumarate:coenzyme A ligase (4CL) isoenzymes in soybean (Glycine max L.). The deduced amino acid sequences reveal several regions of extended sequence identity among 4CLs of all plants analyzed to date. The sequences of two of these regions are consistent with a domain structure proposed for a group of enzymes catalyzing the ATP-dependent covalent binding of AMP to their substrates during the reaction sequence. By using two cDNA fragments that do not cross-hybridize under the conditions used, we demonstrate that 4CL in soybean is very likely encoded by a small gene family. Members of this family are differentially expressed in soybean cell cultures treated with beta-glucan elicitors of Phytophthora megasperma f. sp. glycinea or in soybean roots infected with either an incompatible or compatible race of the fungus. These results are in agreement with our previous observation that elicitor treatment of soybean cells caused a preferential enhancement in the activity level of one of the 4CL isoenzymes. In soybean, 4CL isoenzymes possessing different substrate affinities for substituted cinnamic acids, and showing differential regulation to environmental stress, may play a pivotal role in distributing substituted cinnamate intermediates at a branch point of general phenylpropanoid metabolism into subsequent specific pathways.