The MMS4 gene of Saccharomyces cerevisiae was originally identified due to its sensitivity to MMS in vegetative cells. Subsequent studies have confirmed a role for MMS4 in DNA metabolism of vegetative cells. In addition, mms4 diploids were observed to sporulate poorly. This work demonstrates that the mms4 sporulation defect is due to triggering of the meiotic recombination checkpoint. Genetic, physical, and cytological analyses suggest that MMS4 functions after the single end invasion step of meiotic recombination. In spo13 diploids, red1, but not mek1, is epistatic to mms4 for sporulation and spore viability, suggesting that MMS4 may be required only when homologs are capable of undergoing synapsis. MMS4 and MUS81 are in the same epistasis group for spore viability, consistent with biochemical data that show that the two proteins function in a complex. In contrast, MMS4 functions independently of MSH5 in the production of viable spores. We propose that MMS4 is required for the processing of specific recombination intermediates during meiosis.