The two members of the watermelon mosaic virus subgroup, WMV-1 and WMV-2, were considered to be different viruses on the basis of host range and serology. Immune electron microscopy with serological differentiation indices of 2 to 3 confirmed this serological difference. Molecular hybridization analysis using complementary DNA prepared by reverse transcription of randomly primed WMV-1 RNA showed no homology between WMV-1 and WMV-2 and the complementary DNA from a range of legume infecting potyviruses and potato virus Y (PVY). Molecular hybridization analysis further confirmed that WMV-1 and WMV-2 are distinct viruses.