In eubacteria, the recA gene has long been recognized as essential for homologous recombination and DNA repair. Recent work has identified recA homologs in archaebacteria and eukaryotes, thus emphasizing the universal role this gene plays in DNA metabolism. We have isolated and characterized two new recA homologs, one from the basidiomycete Coprinus cinereus and the other from the angiosperm Lycopersicon esculentum. Like the RAD51 gene of Saccharomyces cerevisiae, the Coprinus gene is highly induced by gamma irradiation and during meiosis. Phylogenetic analyses of eukarotic recA homologs reveal a gene duplication early in eukaryotic evolution which gave rise to two putatively monophyletic groups of recA-like genes. One group of 11 characterized genes, designated the rad51 group, is orthologous to the Saccharomyces RAD51 gene and also contains the Coprinus and Lycopersicon genes. The other group of seven genes, designated the dmc1 group, is orthologous to the Saccharomyces DMC1 gene. Sequence comparisons and phylogenetic analysis reveal extensive lineage- and gene-specific differences in rates of RecA protein evolution. Dmc1 consistently evolves faster than Rad51, and fungal proteins of both types, especially those of Saccharomyces, change rapidly, particularly in comparison to the slowly evolving vertebrate proteins. The Drosophila Rad51 protein has undergone remarkably rapid sequence divergence.