THE disease of tomatoes caused by spotted wilt virus often shows considerable variation in severity. D. O. Norris has now shown (Bull. 202, Coun. Sci. and Ind. Res., Australia ; Melbourne, 1946) that this is due to varying combinations of at least five strains of the virus. The paper describes and figures the symptoms of these strains on sixteen host species, and indicates suitable tests for the presence of strains in a particular disease. The thermal death-point of spotted wilt complexes is considered to be about 10° C. higher than the usual value of 42° C. This is a rather unwelcome discovery from the point of view of possible control. On the other hand, the finding of a high degree of resistance to spotted wilt by Lycopersicum peruvianum is likely to be of considerable importance in breeding.