Cytokinins influence several fundamental processes of plant growth and development, including cell division and organogenesis. To identify genes involved in cytokinin response, a screen was carried out for mutants of Arabidopsis thaliana (L.) Heynh. that are resistant to elevated levels of exogenous cytokinins. One such mutant was isolated and named cyr1. A cross to another Arabidopsis cytokinin-resistant mutant, ckr1 (Su and Howell 1992), indicated that the two mutants are not allelic. The recessive cyr1 mutation causes tenfold decreased sensitivity to benzyladenine in a root-elongation assay but does not confer resistance of roots to indole-3-acetic acid, the ethylene precursor 1-aminocyclopropane-1-carboxylic acid, or abscisic acid. The mutant has increased sensitivity to abscisic acid. This mutation is apparently pleiotropic, giving rise to shoot abnormalities. The phenotype of the cyr1 shoot includes abbreviated development with reduction in cotyledon and leaf expansion, limited leaf production, reduced chlorophyll accumulation, failure to accumulate anthocyanins in response to cytokinins, and the formation of a single infertile flower. These traits, as well as root resistance to cytokinin, are all consistent with a defect in cytokinin action, and are probably due to mutation of a single gene. The cyr1 gene is located on chromosome 5, between 76.2 and 77.6 cM.