A variant of zucchini yellow mosaic virus ZYMV (ZYMV-WK) that induced mild foliar mottle but not fruit symptoms on cucurbits was selected from a poorly aphid-transmissible isolate that induced severe symptoms. This mild variant could not be differentiated from its originating isolate serologically, by its host range, or by aphid transmissibility. ZYMV-WK reduced the marketable fruit production of zucchini squash only slightly when plants were infected at the seedling stage. In field trials conducted in France in 1988 and 1989, ZYMV-WK proved to be very efficient in protecting two cultivars of zucchini squash from severe ZYMV under intense disease pressure (.)