About one-half of the ribosomal repeat unit of two isolates of Pythium ultimum was amplified by means of the polymerase chain reaction using one primer pair. The amplified region includes a small part of the large subunit ribosomal RNA gene, about half of the small subunit ribosomal RNA gene, and the entire intergenic region. The intergenic region of both isolates of P. ultimum has length heterogeneity due to the presence of subrepeat arrays (Klassen and Buchko 1990). PCR amplification of the heterogeneous target DNA resulted in sets of fragments which accurately reflect the heterogeneity in the target DNA, although there is a preferential amplification of the smaller targets. PCR product sizes ranged from 4.6 to 5.8 kb.