Ultra-violet radiation of certain strains of Glomerella cingulata that usually produce perithecia but rarely called forth immediate and profuse production of perithecia. Radiation was by a Cooper-Hewitt quartz mercury arc of 4.5 amp and 66 volts at 21 cm. distance, and from 1/4 sec. to 1 1/2 min. The effective wave lengths range from 313 m[mu] to 180 m[mu]. Different strains of Glomerella reacted very differently to radiation. In Coniothyrium pycnidial formation was stimulated by appropriate radiation.