Diageotropica (dgt) is a single gene mutant of tomato which requires extremely low concentrations of ethylene for normal growth and development. The morphology of the mutant is charactertized by diageotropic growth of both shoots and roots, thin stems without large secondary xylem vessels but with abnormally thick phloem fibers, dark green hyponastic leaf segments, primary and adventitious roots without lateral branching, and an open hypocotyl hook. Studies with the mutant indicate that plant morphology may be at least partly controlled developmentally by an endogenous ethylene–auxin feedback mechanism. The mutant's requirement for ethylene concentrations in the range of 0.005 μl/liter or less implies that higher concentrations may be supraoptimal in studies dealing with ethylene effects on plant growth and development in tomato. Results of grafting and excised root culture experiments combined with results of ethylene treatments indicate that a polarly transported substance, which controls lateral root initiation but not adventitious root initiation, is produced in the shoot in response to ethylene and transported to the roots. The dgt mutant should prove useful in investigations of ethylene and auxin control of morphogenesis.