A spontaneous mutant, Abs(-), that does not abscise any organs despite an apparently normal pattern of growth and senescence was isolated from among plants of Lupinus angustifolius cv. 'Danja'. Abs- was found to be a recessive single gene mutation, and it was proposed that the gene for the original mutant phenotype, referred to as Abs(-), be designated abs1. An artificially induced mutant allelic to abs1 was also obtained and a non-allelic mutant phenotype, Delabs (delayed abscission), which was designated abs2. Morphological and cytological features of the abscission process under conditions of natural and ethylene-induced senescence were compared in the wild-type parent and Abs- mutant. In the parent genotype abscission under natural conditions is similar to many other species, consisting of a stage of cell division forming an abscission zone, activation of the cytoplasm of zone cells, dissolution of the middle lamella, disorganization of fibrillar wall structure, and cell separation. A slightly different pattern of abscission zone development was observed for ethylene-treated explants of the parent, mainly with respect to features of cell division and cell enlarge ment. In Abs- no abscission occurred for any abscission sites under conditions of natural senescence or with ethylene treatment of small shoot explants. However relatively normal abscission zones were differentiated at all sites in the mutant except that extensive cell walt disorganization did not occur. Ethylene production by leaves or other organs of the mutant was no different from that of Danja. Application of copper salts or hydrogen peroxide, droughting, waterlogging, or application of abscisic acid (ABA) increased ethylene production equally in both genotypes but did not result in abscission in the mutant. Release of root cap border cells, the only other cell separation process examined, was similar in each genotype. The study concludes that the mutation is quite specific to the abscission process and may be due to a lack of or delay in the expression of hydrolytic enzyme(s) associated specifically with abscission zone differentiation and separation.