In August 1996, a severe viral disease occurred in squash produced in the subtropical Province of Salta, Argentina. Plants of Cucurbita pepo L. (zucchini) and Cucurbita maxima L. ‘Any’ were affected. Approximately 50% of the plants showed prominent yellow mosaic, necrosis, and foliar distortion. Most of the fruits on infected plants were small, with scattered glossy yellow knobs over a green background, and some showed additional fruit malformation. A potyvirus was isolated from infected plants by mechanical transmission. Filamentous particles were associated with symptomatic inoculated squash and cucumber plants in leaf-dip preparations with an electron microscope (Elmisckop I, Siemens, Germany). The particles were flexuous rods ≈755 nm long. The host range of the isolated virus was mostly limited to the cucurbits (systemic infection) but also included Gomphrena globosa (local and systemic symptoms) and Chenopodium quinoa and Chenopodium amaranticolor, which showed only local chlorotic lesions. Twenty species in the Compositae, Leguminosae, and Solanaceae were not infected (2). In agar double-diffusion tests with sodium dodecyl sulfate (SDS)-degraded virus particles, no reactions were observed with Papaya ringspot virus, Watermelon mosaic virus (WMV), and WMV-Mo antisera, but a strong precipitin line was obtained with the antiserum prepared by Purcifull (1) for isolate 1119 of Zucchini yellow mosaic virus (ZYMV). Isolate 1119 and our isolate appeared serologically indistinguishable in SDS immunodiffusion tests. Both gave fused precipitin bands without spur formation. The results (field symptoms, transmissibility, host range, particle morphology, and serology) lead to the conclusion that ZYMV is present in Argentina. Since 1996, outbreaks have occurred every year in Salta, devastating squash crops. In 1998, ZYMV also was found in the provinces of Mendoza and San Juan, infecting zucchini squash and melons (Cucumis melo L.). A survey of crops indicates that disease incidence and severity were lower than in Salta. References: (1) D. E. Purcifull et al. Plant Dis. 68:230, 1984. (2) H. L. Wang. Plant Dis. 76:530, 1992.