Trichomes, specialized cells that form on the above ground parts of plants, are useful model systems for studying cell differentiation. In this study, the plant hormone ethylene was found to strongly inhibit formation of trichomes on stems of Arabidopsis thaliana. Plants grown in the presence of high concentrations of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid fail to form trichomes on their primary inflorescences. In addition, plants carrying mutations in CTR1 that confer a constitutive response to ethylene exhibit severe reductions in stem trichome numbers. In contrast, plants carrying mutations that confer ethylene insensitivity, and plants grown in the presence of an ethylene biosynthesis inhibitor, produce normal numbers of stem trichomes. Together, these results suggest that either excess ethylene or a constitutive ethylene response prevents the normal differentiation of cells that would otherwise form stem trichomes. Reduced ethylene levels and decreased ethylene response, in constrast, appear insufficient to cause cells that do not normally form trichomes to form trichomes. In contrast to ethylene, application of exogenous Glc results in increased stem trichome numbers. Besides affecting stem trichome numbers, ethylene may also affect branching of stem trichomes. In Arabidopsis thaliana, the vast majority of stem trichomes are unbranched. When wild-type Arabidiopsis thaliana of the Col-0 ecotype are grown in the presence of an ethylene biosynthesis inhibitor, the percentage of stem trichomes that are branched increases significantly. However, growth in the presence of an ethylene biosynthesis inhibitor does not affect stem branching in wild-type Arabidopsis thaliana of the Ler-0 ecotype. Plants carrying the etr1-1 and ein2-1 mutations, which cause ethylene insensitivity, have an increased percentage of branched stem trichomes. In contrast, plants carrying the ctr1-1 and ctr1-12 mutations have a decreased percentage of branched stem trichomes. Growth in the presence of a precursor of ethylene biosynthesis also causes a substantial reduction in branching of Arabidopsis leaf trichomes, suggesting that ethylene has a negative effect on branching of both leaf and stem trichomes in Arabidopsis.