A new yellowing disease of lettuce, sugarbeet, carrot, and other crop and weed hosts was found in the desert areas of southwestern United States. The inciting virus (lettuce infectious yellows virus [LIYV]) was transmitted by the sweet potato whitefly (Bemisia tabaci)in a semipersistent manner, but it was not mechanically transmissible. The virus was retained by viruliferous whiteflies for 3 days in serial transfers on susceptible hosts. LIYV had a wide host range (45 species in 15 plant families) and caused economically significant losses in a number of important crop plants. The virus was purified by differential centrifugation and density gradient centrifugation. Purified preparations had an A260/280nm ratio of 1.28 and contained long flexuous particles 13-14 nm wide and 1,800-2,000 nm long. An antiserum with a homologous titer of 1/1,024 showed no relationship to beet pseudo-yellows virus and could be used to detect greenhouse- and field-infected plants by the ELISA method. The host range, particle size, insect transmission, and serology clearly distinguished LIYV from previously described viruses.