Nitrosoguanidine was used as a mutagenic agent to obtain 4 constitutive and 20 temperature-sensitive mutations of Phyllosticta maydis for increased virulence on a single host line. The temperature-sensitive mutants were of three types: those temperature sensitive on agar and not on the host, those temperature sensitive on the host and not on agar, and those temperature sensitive on both agar and the host. These mutations for increased virulence are interpreted to indicate that host–parasitic specificity is for incompatibility, and that increases in virulence involve selected losses in function, rather than gains. The pattern of temperature sensitivity of certain mutants indicates that avirulence may be one pleiotrophic function of genes which otherwise function in support of basic life processes for the organism.