Meiotic chromosomes are organized about a proteinaceous core that forms between replicated sister chromatids. We have isolated a Caenorhabditis elegans gene, him-3, which encodes a meiosis-specific component of chromosome cores with some similarity to the yeast lateral element protein Hop1p. Antibodies raised against HIM-3 localize the protein to condensing chromosomes in early prophase I and to the cores of both synapsed and desynapsed chromosomes. In RNA interference experiments, chromosomes appear to condense normally in the absence of detectable protein but fail to synapse and form chiasmata, indicating that HIM-3 is essential for these processes. Hypomorphs of him-3, although being synapsis proficient, show severe reductions in the frequency of crossing-over, demonstrating that HIM-3 has a role in establishing normal levels of interhomolog exchange. Him-3 mutants also show defects in meiotic chromosome segregation and the persistence of the protein at the chromosome core until the metaphase I-anaphase I transition suggests that HIM-3 may play a role in sister chromatid cohesion. The analysis of him-3 provides the first functional description of a chromosome core component in a multicellular organism and suggests that a mechanistic link exists between the early meiotic events of synapsis and recombination, and later events such as segregation.