1. Four main types of interaction between paired mycelia of Coprinus lagopus have been defined in terms of the extent of nuclear migration.2. Nuclear migration was demonstrated in matings between monokaryotic mycelia with common A alleles. No extensive migration of nuclei was found in common B or common AB matings.3. The speed of nuclear migration in common A matings was slower than in compatible matings. Migration occurred to approximately the same extent in both kinds of matings.4. In heterokaryon-monokaryon matings compatible and common B heterokaryons acted only as donors. Common AB heterokaryons acted as donors and as acceptors of compatible or common A nuclei. Common A heterokaryons always acted as donors and frequently acted as acceptors of compatible or common A nuclei.5. A simple explanation is suggested for the frequently observed fact that in an incompatible di-mon mating both nuclei of the dikaryon may migrate and eventually eliminate the nuclei of the established monokaryon.