Hypertrophy during the uredial stage of bean rust (Uromyces phaseoli) on Phaseolus vulgaris was manifested by concavities or convexities in the areas of localized light or heavy inoculations of young primary leaves; by measurable increases in the host cell dimensions in the region of localized hypertrophy; by visible increases in the total growth of infected primary leaves when compared with their respective healthy opposite leaves; and as measurable increases in total growth of leaves when entire plants were inoculated. Stimulation of leaf growth by rust infection was detected in as little as 12 hrs. after inoculation. The degree of hypertrophy depended on the infection level. No evidence of hyperplasia was observed.