Abstract
After multiple sap transmissions two isolates of tomato spotted wilt virus (TSWV) lost the characteristic particles, and amorphous, more or less diffuse masses were observed in the cytoplasm of cells. The amorphous material, which had previously been described associated with normal virus particles, could also be found in pellets from extracts of infected plants. Isolates of the amorphous type had the same thermal inactivation point as those of the normal particulate type. It is suggested that the amorphous type is a defective form of TSWV.