SUMMARYMelon necrotic leaf spot virus (MNSV) caused a major outbreak of a leaf necrosis disease of hydroponically‐grown cucumber plants at Humberside in 1983. The virus hadc. 33 nm diam. particles which reacted serologically with MNSV antiserum of Dutch or American origin. Virus particles, which contained a single polypeptide (mol. wt 45 × 103) and a presumed RNA species (mol. wt 1.5 × 106), had a sedimentation coefficient (s20.w) of 134 S and a buoyant density in caesium chloride of 1.35 g/cm3. The virus was mechanically transmissible, confined to species ofCucurbitaceae, transmitted by zoospores ofOlpidium radicaleand retained in the resting spores of the fungus. MNSV is thus both water‐borne and soil‐borne.O. radicalezoospores were killed in<5 min in suspensions containing 20 μg/ml of the surfactant Agral (alkyl phenol ethylene oxide). The disease did not reappear in 1984 when the cucumber crops were fed with nutrients containing 20μg