Phycomyces mutants, recently isolated for enhanced bending responses (hypertropic phenotype), have unusual genetic properties. In sexual crosses between hypertropic mutants and other strains, the progeny showed the following features: a) many incomplete tetrads, b) distortion of segregation ratios, c) progeny with nonparental phenotypes when hypertropic strains carrying mutations in the same gene or even the same allele were crossed, and d) morphologically abnormal progeny with phenotypes unrelated to those of the parents. In particular, the mesophorogenic colonies, which produced short sporangiophores, were genetically unstable; their mycelia produced sectors with normal morphology and segregated several alleles for different markers. Most of the phenomena (mutation, segregation distortion, and sterility) described in this paper resemble the "hybrid dysgenesis" syndrome in Drosophila. The results suggest that all seven hypertropic mutations affect the process of meiosis and thereby lead to unstable aneuploid progeny.