Abstract The mei-218 gene product is required for both meiotic crossing over and for the production of recombination nodules, suggesting that these organelles are required for meiotic exchange. In this study the null phenotype of mei-218 was defined through the analysis of three preexisting and five new alleles. Consistent with previous studies, in homozygous mi-218 mutants meiotic crossing over is reduced to <10% of normal levels. A molecular analysis of mei-218 was initiated with the isolation and mapping of lethal mutations and genome rearrangements in the region containing mi-218, polytene interval 15E on the X chromosome. This high resolution genetic map was aligned with a physical map constructed from cosmid and P1 clones by genetically mapping restriction fragment length polymorphisms and localizing rearrangement breakpoints. Within a region of 65 kb, we have identified seven transcription units, including mei-218 and the Minute(l)l5D gene, which encodes ribosomal protein S5. The mei-218 mutant phenotype has been rescued by germline transformation with both a genomic fragment and a cDNA under the control of the hsp83 promoter. The mei-218 gene is predicted to produce an 1186 amino acid protein that has no significant similarities to any known proteins.