During the tenure of this fellowship, I intend to contribute to a genetic dissection of the development of the fungus Phycomyces blakesleeanus, drawing from my experience with mutations affecting the development of Drosophila melanogaster. This project should lead to the first complete formal genetic description of indispensable ontogenetic mechanisms in a fungus and lay the basis for their explanation at the molecular level. We plan to isolate and characterise variants of all the stages that have been recognised morphologically in the asexual and sexual life cycles of Phycomyces. Some of these stages may be completely defined by viable mutants. Lethal mutations in other steps can be detected, maintained and genetically analysed as heterokaryons. Homokaryons exhibiting a lethal phenotype can be selected, with the help of available markers, among random segregants formed when the heterokaryons grow old or sporulate. Some lethal blocks may be avoided by developmental shortcuts: thus the regeneration of mycelia from macrophores and old propagula would circumvent spore formation and spore germination, respectively.