The relationship of the Morocco isolate of watermelon mosaic virus (WMV) to WMV2, soybean mosaic virus (a virus closely related to WMV2) and the W strain of papaya ringspot virus (PRSV-W), formerly WMV1, was examined by comparing tryptic peptide profiles using high performance liquid chromatography. The profiles indicated that the coat protein sequence of WMV-Morocco differed substantially from those of the other potyviruses. This conclusion was supported by sequence data from five tryptic peptides from the coat protein of WMV-Morocco, which showed only 61-68% identity to equivalent sequences in PRSV-W, WMV2 and zucchini yellow mosaic, another potyvirus infecting cucurbits. Based on the above data, and on known correlations between coat protein sequence similarities and potyvirus relationship, it is concluded that WMV-Morocco should be regarded as a distinct potyvirus.