A range of mutants with impaired ability to infect tomato cotyledons and leaves was produced by exposing conidia of Fulvia fulva (race 4) to either u.v. light or 1,3-butadiene diepoxide or N-methyl-N-nitro-N-nitrosoguanidine. One spontaneous mutant was also recovered. In axenic culture, growth rates and sporulation of the mutants were similar to the wild-type race 4. Two classes of mutant could be detected by tests on seedlings. One class, designated pathogenicity mutants (Pat-), failed to sporulate; the other, designated aggression mutants (Agg-), showed reduced sporulation. No mutants for virulence (i.e. changed specificity for differential host lines) were obtained. Light and scanning electron microscopy revealed that the Pat- mutants could be further divided into three categories, Pat-A, Pat-B and Pat-C. Hyphae of Pat-A mutants penetrated the stomata then stopped, Pat-B mutants invaded the mesophyll but grew sparsely, while Pat-C mutants grew densely in the mesophyll. Microscopic observations were more sensitive than chitin or ergosterol assays for distinguishing mutants by growth in planta. Callose deposition in response to infection was proportional to the amount of growth by each category of mutant. Accumulation of pathogenicity-related (PR) proteins in intercellular fluid showed similar relationships. Compared with wild-type races, Pat-C mutants induced a similar range and level of PR proteins but these accumulated less rapidly, whereas Pat-B mutants induced only low levels and Pat-A mutants did not induce accumulation of PR proteins.