Field incidence of a virus disease of zucchini squash causing severe mosaic, vein banding, shoe stringing, distortion of leaves and fruits with occasional blisters was monitored during 1988–1990 at 12 locations in four districts of Nepal. The causal agent was transmitted by sap and aphids but was not seed‐borne. Electron microscopic observation of samples collected from infected plants revealed flexuous elongate particles resembling those of potyviruses. Based on these results, the causal virus was tentatively diagnosed as zucchini yellow mosaic virus. In field plantings the onset of the disease was earlier and spread occurred more rapidly in March and April plantings than in one made in January. Spread was more than 8.5% per day during both pre‐ and post‐monsoon seasons. The disease significantly reduced various yield attributes. The disease was epidemic in most farmers’ fields at low elevations (about 250 m) and much less at higher altitudes (>1500 m).