We report plaquing tests of 124 virus strains, mostly arboviruses of 21 serological groups, in Singh's line of Aedes albopictus cells. Thirty of these plaqued; all were arboviruses of six groups and were known or presumed to be mosquito borne. Failing to plaque were 86 strains of arboviruses, mostly tick borne, two strains of insect pathogens, and six animal viruses not classified as arboviruses. Among mosquito-borne agents, plaquing ability appeared related to serological classification. California group and most A-group viruses failed to plaque, but nearly all members of B and Bunyamwera groups readily plaqued. Within serological group B, 14 of 16 mosquito-borne agents plaqued, but none of 13 tick-borne or vector-unassociated viruses did so. Some implications of these results for recognition and classification of arboviruses are discussed.