The expression of the agrobacterial iaaM gene for tryptophan monooxygenase, the enzyme catalyzing the first step in the auxin biosynthesis, induced substantial physiological and biochemical changes in transgenic tobacco (Nicotiana tabacum L.) plants. All lines of transgenic plants grown in vitro manifested abnormal phenotypes: enhanced root formation, adventitious roots on stems, and curled leaves. When grown in vivo, plants manifested abnormal, normal, or intermediate phenotype. Under conditions of a greenhouse, the abnormal plants contained the highest amount of auxins in their leaves and manifested an increased number of adventitious roots, poor reproductivity, and the loss in seed germination. Transgenic plants with the normal phenotype did not substantially differ from the wild-type plants in their morphology, and their auxin content was lower than in the abnormal plants. The intermediate-phenotype plants were devoid of some morphological properties characteristic of the abnormal plants. Only the seeds of normal- and intermediate-phenotype transgenic plants germinated at a high rate.