Abstract A description is provided for Microcyclus ulei . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Restricted to Hevea spp. Recorded on: H. brasiliensis (Para rubber tree), H. benthamiana, H. guianensis and H. spruceana . DISEASE: South American leaf blight of Para rubber. Greyish, powdery conidial lesions on the leaves followed by the stromatic stage 3-6 weeks later. Infection of very young leaves, up to 6-10 days old, leads to leaf fall, branch dieback and death of well grown trees. Symptoms typically on leaves but also on green stems, petioles, inflorescence, flowers and young fruit. One of the most destructive diseases of rubber and a factor limiting the development of a natural rubber industry in America. It prevented attempts to establish the crop in America in the 1930's and 1940's (35: 716). GEOGRAPHICAL DISTRIBUTION: Restricted to tropical South and Central America, including the Sao Paulo area of Brazil and Trinidad (CMI Map 27, ed. 4, 1967). Endemic on wild species of Hevea in the Amazon valley, Guiana, Madre de Dios and Matto Grosso regions of South America. TRANSMISSION: Air-borne conidia which have a diurnal periodicity with a forenoon maximum and minima at night and early morning. Conidia can be spread on the surface of water droplets. Ascospores presumably also air-borne but their role in the epidemiology is unknown (46, 1089f; 48, 270e).