Abstract A description is provided for Melampsora larici-populina . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Pycnia and aecia on Larix , especially L. decidua and L. leptolepis . Uredinia and telia on Populus spp., especially P. nigra and its varieties and hybrids, P. balsamifera, P. yunnanensis and others. Its exact host range on Populus spp. is not clear due to confusion with other species of Melampsora and to uncertainty in the reported identity of some species of Populus and clones. DISEASE: Leaf rust of poplars, causing severe leaf damage and early defoliation on susceptible hosts. Defoliation can sometimes occur two or three times in the season, each defoliation being followed by a new growth flush. These successive defoliations lead to winter injury and dieback (Peace, 1962) and a reduction in incremental timber growth can occur. Rust damage also renders some varieties more susceptible to Dothichiza dieback (39, 57) caused by Cryptodiaporthe populea (CMI Descript. 364). Poplars vary greatly in their reaction to infection and some clones show useful resistance (39, 57; 42, 347; 43, 498, 551; 47 129; Steenackers, 1972). Seasonal conditions may influence the reaction of some varieties (45, 402). On larch ( Larix ) hosts, the damage is negligible (Peace, 1962). GEOGRAPHICAL DISTRIBUTION: Africa (North Africa, Rhodesia, South Africa), South America (Argentina, Chile, Colombia); Asia (Israel, Korea, Japan), Australasia and Oceania (Australia, New Zealand), Europe (widespread; reported specifically for Britain, Denmark, Finland, France, Germany, Ireland, Italy, Lithuania, Netherlands, Norway, Portugal, Spain, Sweden, USSR). TRANSMISSION: By air-borne urediniospores, often over long distances (suspected from eastern Australia to New Zealand). In Japan, urediniospores produced in autumn can remain viable and pathogenic until the following spring (41, 745), and in milder climates they can survive on semi-evergreen lines and on green sucker growth of deciduous trees. This is the main method of overwintering in Australia. In the Northern Hemisphere, overwintered telia give rise to basidia and basidiospores which may cause inconspicuous spring infections on Larix.