Industrial yeast strains carry one of two homeologous 2 microns plasmids designated as type-1 or type-2. The 2 microns plasmid, Scp1, found in common laboratory strains of Saccharomyces cerevisiae is considered a type-2 plasmid, since the ori, STB, RAF and REP1 loci and intergenic sequences of the right-unique region of Scp1 are homologous to the corresponding loci in industrial strain type-2 plasmids. However, within both its 599 bp inverted repeats Scp1 has 142-bp sequences homologous to the bakers' yeast type-1 plasmid. DNA sequence analyses and oligonucleotide hybridizations indicate that the 142-bp insertion in Scp1 was probably due to homeologous recombination between type-1 and type-2 plasmids. These results suggest that some of the plasmid and chromosomal sequence polymorphisms seen in laboratory yeast strains result from homeologous recombination in their ancestral breeding stock.