Seed lots prepared from fruits collected from systemically infected melon plants were tested for seedborne melon necrotic spot carmovirus (MNSV). The hypothesis of "vector-assisted" seed transmission (VAST) was tested in trials with rigorous control of the vector fungus, Olpidium bornovanus. The vector also formed the basis for the most sensitive assays of seed-borne virus, enzyme-linked immunosorbent assay (ELISA) was intermediate, and infectivity was the least sensitive assay. Although extracts of seeds not treated with acid reacted in ELISA, the assay did not discriminate between viral antigens and infectious virus. The superficial viral antigens and virions were removed by acid treatment of seeds, but infectious virus remained within the seed coat at levels not usually detectable by ELISA. Seedborne virus rarely infected seedlings unless the vector was present, confirming VAST as a novel means of seed transmission that is possible because of the in vitro method of virus acquisition by O. bornovanus. The incidence of internally borne MNSV ranged from 0.1 to 5.3% in the vector-based assays. In one seed lot, MNSV was primarily on the surface of seeds, with an incidence of >50% on nontreated seeds and 0% on acid-treated seeds.