Xanthomonas citri XCJ19, infected with temperate phage PXC7, had its smooth colony cells altered to produce dwarf colonies. The altered dwarf clones were lysogenic. After incubation in nutrient broth for 72hr, each of them reverted in 0.1-6% of its cells to produce smooth colonies, among which 60% were lysogenic, 25% resistant, and 15% sensitive. The lysogenic dwarf convertants could not be recovered to produce smooth colonies by supplements of amino acids, vitamins, or nucleic acid bases; however, they lysed and liberated spontaneously phage PXC7 at a considerably higher frequency and decreased the growth rates and the number of bacterial cells contained in the colonies, as compared with the lysogenic smooth revertants. This indicates that the lysogenic dwarf convertants have arisen from Xanthomonas citri XCJ19 as the result of the extraordinary high-frequency of spontaneous induction of the phage after lysogenization. The lysogenic dwarf convertants, though they exhibited characteristics similar to the smooth original strain on most of the tests applied, showed changes in cell disposition and response to virulent phage CP₁.