The Dictyostelium discoideum mutant MC2 exhibits temperature-sensitive growth, temperature-sensitive motility, and temperature induction of discoidin I synthesis. These three phenotypes of MC2 were not separated in the genetic experiments reported here. They were therefore assigned to the mutation motA1552, which was mapped to linkage group II by segregation analysis and by analysis of mitotic recombinant diploids. In one motA1552 strain, loss of motility preceded accumulation of discoidin I by 3 hr, indicating that discoidin I is not involved in generation of the motility defect. Expression of motA1552 phenotypes varied both among strains carrying the mutation, and among different clones of a particular strain. MC2 and its derivatives displayed elevated levels of recombination between whiA and acrA on linkage group II, and yielded highly unstable mutations at the acrA locus. Accumulation of large amounts of discoidin I during axenic growth of strain AX3 was found to depend on the presence of a second linkage group II mutation, daxA1551. This mutation was already present in the strain mutagenized to isolate motA1552, complicating explication of motA1552 action.