Abstract: The multicopy yeast plasmid, 2mu circle, encodes a specialized recombination system. It contains two regions, each 599 bp in length, that are precise inverted repeats of each other and between which recombination occurs readily. In addition, this recombination requires the product of a 2mu circle gene, designated FLP. By examining the products of FLP-mediated recombination of plasmids containing single insertions within one of the repeated regions, we show that this recombination occurs only at a specific site within the repeat. This result was confirmed from analysis of the ability of plasmids containing various deletions within one of the repeated regions to serve as substrates for FLP-mediated recombination. These experiments limit the recombination site to a sequence of less than 65 bp. In addition, by mutational analysis of the recombination potential of a hybrid plasmid containing the entire 2mu circle genome, we have shown that FLP is only the 2mu circle gene necessary for this site-specific recombination. Finally, we describe a sensitive assay for recombination between the repeated sequences of 2mu circle; using it, we demonstrate that even in the absence of FLP gene product, recombination between the repeats occurs at a low but detectable level during meiosis.