The RAD50 gene of S. cerevisiae is required during meiosis for both recombination and chromosome synapsis and is also required for repair of double strand breaks during vegetative growth. We present below the isolation and analysis of several types of rad50 mutants. We show that null mutations block both meiotic recombination and formation of synaptonemal complex (SC) at early stages, while nonnull mutations block both processes at intermediate stages. These observations suggest that recombination and SC formation involve a series of intimately related events. Furthermore, all rad50 mutants block formation of tripartite SC structure but permit other aspects of SC development, i.e., formation of axial cores. In light of this and other observations, the meiotic and mitotic defects of rad50 mutants can be accounted for economically by the proposal that meiotic recombination, meiotic chromosome pairing, and vegetative DNA repair all use a common chromosomal homology search that involves RAD50 function.