Corn stunt in São Paulo State is usually of minor importance during normal summer crops but might induce significant losses if corn is planted late in the season. It is transmitted by leaf - hopper Dalbulus maidisDe L. & W. and two different forms of this disease have been recognized the chlorotic, similar to the Rio Grande type of the U.S., and the red, resembling the Mesa Central type of Mexico. Electron microscopic examination of tissues from both affected plants or leafhopper reared on diseased plants, demonstrated the presence of pleomorphic, mycoplasma-like bodies, consistently associated with the corn stunt, which probably represent the causal agent the latter. No significant difference could be observed in the morphology of these mycoplasma-like bodies associated with the two disease types or with the insect vector. In the affected plants, these corpuscles were only found in the sieve tubes; in the insect body they were detected in several types of tissues of different organs - intestine, Malphigian tubules, muscle, nervous ganglion, salivary gland, fat body, epidermis. In the insect cells, the pleomorphic bodies occurred either scattered in the cytoplasm or in cytoplasmic cavities bounded by membrane.