Summary  SDS-PAGE of seed proteins from the seeds of a nondomesticated bean of Mexican origin (Phaseolus vulgaris L., PI 325690) revealed the presence of a novel 38 kd protein which appeared to be neither an altered phaseolin nor a lectin fraction. The protein was named arcelin, after Arcelia, the town in the state of Guerrero near which PI 325690 had been collected. The pure line, UW 325, was derived by self fertilization of the plant from a single arcelin-containing seed of PI 325690. Despite a low percentage seed phaseolin (14.6%), seed phenotype, seed germination, plant growth, pollen fertility, and percentage seed protein of UW 325 were normal. Analyses of F₂ and F₃ seeds from a single F₁ plant of the cross SanilacXPI 325690-3 revealed that arcelin expression was inherited as a single gene and that presence was dominant to absence of arcelin. The mean percentage phaseolin in the seeds of homozygous dominant Arc/Arc F₃ families (14.0%) was significantly lower than that of the homozygous recessive arc/arc seeds (44.7%). The distribution of percentage phaseolin values for seeds within segregating families was bimodal and nonoverlapping. Without exception, seeds containing arcelin (Arc+phenotype) contained a lower percentage phaseolin than seeds lacking arcelin (Arc-phenotype). Although arcelin presence was associated with low percentage phaseolin, the Arc/Arc and Arc/arc genotypes were similar for seed weight and percentage total seed protein.