A not yet identified virus (SMoV) associated with the strawberry mottle syndrome was mechanically transmitted from infected Fragaria vesca UC-5 to Chenopodium quinoa and from C. quinoa to Nicotiana benthamiana, N. occidentalis 37B and N. hesperis 67A. C. quinoa, N. benthamiana, and N. occidentalis 37B developed systemic mosaic symptoms, whereas N. hesperis 67A produced local lesions. In crude extracts of these plants, isometric particles 28 nm in diameter were observed, sometimes in arrays within tubules. However, the presence of these particles was always erratic. Purification of the intact isometric virus particles from C. quinoa or N. occidentalis 37B when leaves were maximally symptomatic was unsuccessful. The infectious agent appeared to be unstable in vitro, could not be separated from the plant material and inconsistent results were obtained no matter the purification strategy followed. N. hesperis 67A has been used as a locallesion host to evaluate the increase of infectivity in N. occidentalis 37B. Maximal infectivity was reached a few days after the onset of the systemic symptoms. Purifications from this plant material gave inconsistent results also. While virus particles in these crude extracts were seldom present, dsRNA-like material was readily revealed by agarose-gel electrophoresis. Infectivity associated with extracts of SMoV-infected leaves of C. quinoa and N. occidentalis 37B was susceptible to low concentrations of RNase. Phenol extracts of infected leaves yielded greater infectivity than buffer extracts of comparable tissue. Infectivity seemed to be associated whith ssRNA.