The relationship between preprophase microtubules and asymmetrical cell division in the formation of the stomatal complex of C. cyanea was investigated. Polarization of nuclei and other cell organelles adjacent to the guard mother cell occurred in most cases without a preprophase band of microtubules being present; the grouping of preprophase microtubules appeared immediately prior to cell division, and its situation, even during abnormal stomatal development, predicted the plane of future division. The results show that preprophase microtubules cannot be the cytoplasmic agents involved in orienting and positioning the nucleus prior to division. Clear evidence was obtained indicating that preprophase microtubules move intact into the spindle. Some aspects of abnormal stomatal development are discussed, and the results are related to some other work on stomatal differentiation.