Poa semilatent virus (PSLV), which caused chlorosis and rapid death of inoculated wheat, infected a number of grasses susceptible to barley stripe mosaic virus (BSMV); but it also infected Agropyron trachycaulum and Poa palustris which were not susceptible to a barley strain or two oat-infecting strains of BSMV. Conversely BSMV caused local lesions on several species of Chenopodium that did not become infected with PSLV. BSMV protected wheat from infection by PSLV, but PSLV caused only partial protection from BSMV.The normal length and thickness of particles in leaf-dip preparations was 161 × 26 mμ for PSLV and 133 × 25 mμ for BSMV.Serological relationship was not indicated by the Ouchterlony agar double-diffusion test or leaf-dip serology. A distant relationship was shown with the microprecipitin test and the ring interface precipitin test. In cross absorption tests the titers of the antisera to the homologous viruses were not reduced by absorption with the heterologous viruses.PSLV and BSMV appear to be distantly related serotypes.