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Alfalfa latent virus (ALV), a new member of the carlavirus group, was isolated From alfalfa (Medicago sarivo) clones from rbe University of Nebraska experimental plots and from the Farmers' fields. The pea aphid, Acvrihosiphon pisum, transmitted this virus to M. sativa, Pisum sativum, and Viria faba. but not to Trifolium pratense. It was also sap transmissible to V. faba, V. villoso, and P. sarivum, but was not seed-borne in P. sariuum. In V. Jaba sap ALV was infectious at 65 but not at 70 C in 10 min. or alter 4 days but not after 6 days at 25 C and after dilution to 10-3 but not 10-4 Light microscopy indicated absence of any inclusion bodies in the epidermal strips of V. faba and P. surivum. With tobacco mosaic virus and a diffraction grating replica as standards, modal lengths of 653 and 635 nm, respectively, were determined for ALV. Serological and host range studies indicate that ALV is unrelated to pea streak virus, rcd clover vein mosaic virus, and cowpea mild mottle virus, other naturally occurring legume carlaviruses.