The biological properties of 4 tobacco mosaic virus (TMV) strains, 8 tomato mosaic virus (ToMV) strains, 1 pepper mild mottle virus (PMMV) strain and 5 cucumber mosaic virus (CMV) strains isolated from crop plants in Bulgaria have been described. TMV, ToMV (the first predominate) and CMV occurred in tobacco. The same viruses were found in pepper, and also the strain P₁₀₁ of PMMV has been isolated from cultivars possessing L gene. ToMV strains only from “0” and “1” Pelham's group and 2 CMV strains - mosaic and necrotic - occurred in tomato. CMV-NB possessing fifth satellite RNA caused yellow mosaic in tobacco, necrosis on tomato stems, leaves and fruits and was attenuated for cucumbers. Virus strains provoking green mosaic with an exception of CMV-NB were typical for the investigated cultures.