An extensive baculovirus epizootic occurred during the autumn of 1976 in a larval population of the clover cutworm,Scatogramma trifolii(Rottenberg), breeding on Russian thistle,Salsola iberica(Sennen and Pau), in southern California. The primary causal agent of the epizootic was a granulosis virus although a nuclear polyhedrosis virus was involved at several locations. The epizootic extended over an area of ca. 1000 km2and included within its limits the suburban Los Angeles cities of Colton, Corona, Fontana, Ontario, Redlands, Riverside, San Bernardino, and portions of Pomona. The incidence of the granulosis disease was greater than 90% at all sites sampled regardless of larval density. This is the 1st report of a granulosis virus fromS. trifolii.