In genetic studies of the haploid fungus Neurospora, strains having chromosome rearrangements of various types are being applied to a variety of problems. Several types (e.g., insertional translocations) recurrently generate via meiosis a class of viable progeny containing nontandem duplications; their properties are of interest for comparison with similar rearrangements in diploid organisms, including man, where direct study is difficult. Duplication-generating aberrations are being used for studying vegetative incompatibility, regulation, complementation and dominance, and mapping by duplication coverage. Many duplications show instability due to chromosome breakage, which is subject to genetic modification. These are being used in a search for control systems that regulate breakage. Acentric chromosome fragments which can replicate are produced by crossing over in inverted transpositions. Their behavior and possible genetic role are being examined. Probably diploids have been obtained, but they are unstable. Conditional lethal mutants are being screened in a search for replication- or repair-defective strains and of chromosome rearrangements that exert a position effect on gene expression. Strains of Neurospora are being collected from nature as a source of genic and chromosomal variants, and analyzed for information on variation and polymorphism.