Dicamba (2-methoxy-3,6-dichlorobenzoic acid) applied to wheat and barley during periods of high meristematic activity disrupted normal growth and induced morphological and cytological aberrations. The intensity and locale of the morphological effects varied with the dosage and time of application. Under field conditions an application of 4 oz per acre before the 4-leaf stage caused gross malformation of the stems and leaves, reduced height, delayed maturity, and interfered with normal seed development in the main culms. Applied during the 4-leaf to 6-leaf stages this dosage interfered with tiller development and markedly reduced seed production in the main culms. Applied during the boot and early headed stages it largely prevented kernel development in both crops. Under lush conditions of the greenhouse, similar effects were induced by half this dosage. Doubling of the rate of application intensified and prolonged these effects to the extent that response differences due to growth stage were largely masked. Root tips of wheat and barley seedlings, germinated in different concentrations of dicamba, showed a sharp reduction in the numbers of dividing cells, viz., 133, 109, 97, 66, and 21 in concentrations of nil, 1, 5, 10, and 100 p.p.m., respectively. However, once initiated, mitosis appeared to proceed in a normal manner. At 10 p.p.m. and upwards, there was much evidence of chromosome clumping and the formation of multinucleate cells. Cell development and arrangement in the florets of treated plants was grossly disturbed.The response pattern of both crops resembled that induced by 2,4-D but the dosage of dicamba required was significantly less.