Recent studies in Saccharomyces cerevisiae have unveiled that meiotic recombination crossovers are formed by two genetically distinct pathways: a major interference-sensitive pathway and a minor interference-insensitive pathway. Several proteins, including the MSH4/MSH5 heterodimer and the MER3 DNA helicase, are indispensable for the interference-sensitive pathway. MSH4 homologs have been identified in mice and Arabidopsis and shown to be required for normal levels of crossovers, suggesting that the function of MSH4 may be conserved among major eukaryotic kingdoms. However, it is not known whether an MER3-like function is also required for meiosis in animals and plants. We have identified an Arabidopsis gene that encodes a putative MER3 homolog and is preferentially expressed in meiocytes. T-DNA insertional mutants of this gene exhibit defects in fertility and meiosis. Detailed cytological studies indicate that the mutants are defective in homolog synapsis and crossover formation, resulting in a reduction of bivalents and in the formation of univalents at late prophase I. We have named this gene ROCK-N-ROLLERS (RCK) to reflect the mutant phenotype of chromosomes undergoing the meiotic 'dance' either in pairs or individually. Our results demonstrate that an MER3-like function is required for meiotic crossover in plants and provide further support for the idea that Arabidopsis, like the budding yeast, possesses both interference-sensitive and insensitive pathways for crossover formation.