1. The occurrence of fusion between plasmodia produced from amoebal clones of P. polycephalum was studied.
2. The occurrence of fusion was found to be strain-dependent and the factors responsible segregated in the progeny of a cross.
3. The segregations found in crosses between several strains led to the conclusion that four alleles (f1–f4) of one gene f were controlling fusion in these strains.
4. Fusion occurs only between plasmodia carrying identical f alleles, except in one class of results.
5. A model accommodating all the results, including the ‘exceptional’ class, is proposed. It requires that the action of the f factors is to inhibit fusion between dissimilar strains rather than to promote fusion between identical strains. Certain physiological deductions from this model are discussed.
6. The locus (mt) determining mating type of the amoebae is not concerned in plasmodial fusion and is unlinked to f.
7. The rate of fusion between some pairs of strains is apparently influenced by modifying genes.
8. It is suggested that, as a result of the operation of the f gene and of the previously described killing reaction, heterokaryons will occur rarely in natural populations of P. polycephalum.