Extensive sampling of naturally infected field and greenhouse tomatoes has revealed the presence of a specialized form of the tobacco mosaic virus. This tomato form, readily differentiated from that commonly occurring in tobacco, has been found in widely separated geographic areas within Canada, and is the dominant, if not the only, form in tomato even in regions where tobacco and tomatoes are grown as contiguous crops. This specialization to tomato of a virus form distinct from that in tobacco does not support the view commonly held that smoking tobacco is the main source of inoculum for the mosaic disease in commercial tomatoes.