Microscopic examination of apple leaves infected by biochemical mutants of Venturia inaequalis showed that all mutants had penetrated the cuticle and had established themselves in the characteristic subcuticular position without an additional source of their required substances from outside the host. Those mutants that were fully as pathogenic as the wild type developed in vivo comparably to it, but those that were nonpathogenic made very limited growth and, with the exception of 1 requiring choline, failed to develop sufficiently to sporulate. However, the nonpathogenic mutants, with the exception of 1 requiring adenine, grew and sporulated in vivo when their required substances were applied to the surface of the inoculated leaves.