A wilt disease of alfalfa caused by Verticillium albo-atrum Reinke et Berthold was detected in most of the alfalfa meadows in Hokkaido. This was the first report of the occurrence of diseases caused by V. albo-atrum in Japan along with the report by Saito (1981) on potato. The fungus which was first introduced from infected seeds from abroad survived in plant residues contaminated during harvesting. Alfalfa cultivars showed different reactions to the pathogen: all the cultivars grown in Hokkaido were susceptible, whereas those selected for resistance were resistant. The disease caused a yield loss of 33% in susceptible cultivars, while the resistant cultivars were not affected. Strains of V. albo-atrum from alfalfa and potato differed in the host range. Preinoculation of alfalfa plants with the potato strain decreased the disease severity.