1. Tomato mosaic may be carried over winter in hothouse tomato crops, but this does not account for the great bulk of mosaic infection in the canning crop. 2. In a total of 22,944 tomato plants grown from seed from mosaic plants, no evidence of seed transmission of the disease was obtained. 3. The mosaic disease has been found occurring in the field on the following perennial weed relatives of the tomato in Indiana: Physalis subglabrata, P. virginiana, P. heterophylla, and Solanum carolinense. Mosaic has been transmitted to tomatoes from each of these species. 4. It has been proved that the mosaic virus persists over winter in the rootstocks of P. subglabrata. The young mosaic shoots appear in the spring before tomatoes are transplanted to the field. From these shoots the disease has been transmitted to tomatoes. 5. Physalis subglabrata, with some admixture of the very similar P. virginiana, is a very prevalent weed in Indiana tomato fields. 6. Examination of these weeds in fields previously in tomatoes s...