Elongated, flexible particles, measuring 15 mμ in diameter, and having a normal length (NL) of about 500 mμ were found in preparations made by dipping method from different host plants infected with cassava common mosaic virus (CCMV), whereas none were seen in non inoculated control plants. Particles with similar morphology were the only components of highly purified and infective preparations. These particles when negatively stained, showed no internal details. Comparative measurements between CCMV and potato virus X (PVX) indicated consistently that the NL of CCMV is about 17 mm shorter than that of PVX. In serological cross reaction tests with antisera to CCMV and PVX of relatively high titre (1/4096) no common antigenic groups between both viruses could be demonstrated. Because of the morphology of the particles, CCMV can be ranged among the PVX relatives of group 4-6 of the classification scheme for elongated plant viruses, as proposed by Brandes and Wetter. The particles of these viruses have been described as flexible threads ranging in normal length from 480 to 580 mμ The slight but detectable difference in morphology, the fact that no common antigenic group could be demonstrated, and the differences in the diseases induced by CCMV and PVX suggest that these viruses are not nearly related. CCMV should be regarded therefore as a distinet entity within the taxonomic group of elongated plant viruses represented by PVX.