A population of M2 seedlings of Arabidopsis thaliana was screened for mutants that were insensitive to the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC). Several independent lines were obtained and proved insensitive to both ACC and ethylene. Two lines were identified as alleles of a single recessive mutation, designated ain1. Linkage analysis indicated that the ain1 gene is located on chromosome 1, adjacent to the cer5 marker and, therefore, genetically distinct from previously identified ethylene resistance loci. General phenotypic aspects of ain1 mutants were similar to wild type. For both alleles, the level of insensitivity to ethylene at the seedling stage was indistinguishable in terms of elongation growth. In contrast, the gravitropic response of ain1-1 seedlings was slower than that of wild-type and ain1-2 seedlings. At the adult stage, stress responses of mutants were similar to wild type. However, ethylene-induced leaf senescence was delayed in both mutants. In addition, we observed significant interallelic variation in ethylene production rates. Growth inhibition experiments showed that the ain1 mutation does not confer resistance to other hormones. Thus, ain1 most probably affects a step specific for the ethylene signal transduction pathway.