Solitary (S) bodies were present in the cytoplasm of the cells of many cultivars of Tropaeolum majus. However, cultivars have also been found which completely lack these S bodies. The S bodies occur in all cells of all kinds of tissues and organs of the plant. In the anthers they occur in the cells of the endothecium tissue as well as in the tapetum layer, and even in the pollen cells. The shape of the S bodies is nearly spherical (diameter ca. 60 nm) with a tail-like appendage with a length of ca. 90-150 nm and a diameter of ca. 16 nm. The nature of the S bodies is unknown. Transmission studies by mechanical inoculation and by grafting gave no indications of a virus nature of these bodies. Reciprocal crosses between plants containing and lacking these S bodies showed that the bodies were transmitted to the progeny only if they occur in the mother plant, that is, indicated a cytoplasmic inheritance.