A syndrome of associated aberrant traits is described in Drosophila melanogaster. Six of these traits, mutation, sterility, male recombination, transmission ratio distortion, chromosomal aberrations and local increases in female recombination, have previously been reported. A seventh trait, nondisjunction, is described for the first time. All of the traits we have examined are found nonreciprocally in F(1) hybrids. We present evidence that at least four of the traits are not found in nonhybrids. Therefore we have proposed the name hybrid dysgenesis to describe this syndrome.-A partition of tested strains into two types, designated P and M, was made according to the paternal or maternal contribution required to produce hybrid dysgenesis. This classification seems to hold for crosses of strains from within the United States and Australia, as well as for crosses between strains from the two countries. Strains collected recently from natural populations are typically of the P type and those having a long laboratory history are generally of the M type. However, a group of six strains collected from the wild in the 1960's are unambiguously divided equally between the P and M types. The dichotomy of this latter group raises interesting questions concerning possible implications for speciation.-Temperature often has a critical effect on the manifestation of hybrid dysgenesis. High F(1 ) developmental temperatures tend to increase the expression of sterility, sometimes to extreme levels. Conversely, low developmental temperatures tend to inhibit the expression of some dysgenic traits.-There are potentially important practical implications of hybrid dysgenesis for laboratory experimentation. The results suggest that care should be exercised in planning experiments involving strain crosses.