SUMMARYA negative correlation has been shown to exist between the growth rate of seedling tomato plants and the length of the incubation period.Evidence has been presented, which suggests that the relative effects oftobacco virus1 on the growth rate of the plant subsequent to the appearance of symptoms is the same in both summer and winter.The percentage water content of all parts of infected plants was found to be lower than that of the controls, during the early stages of the disease. Later the water content tended to rise above that of healthy plants.In conclusion we wish to express our thanks to Mr B. D. Bolas for his interest and valuable advice.