Cyclic and obligate parthenogens of 'Daphnia pulex ' were isolated from populations in Southern Ontario and Michigan and used to survey variation in the intergenic spacer region (IGS) of ribosomal DNA (rDNA). A total of 24 distinct IGS haplotypes was identified from eight isolates used in this study. Mean number of haplotypes per isolate was lower for obligate isolates than for cyclic isolates, suggesting that there is a net loss of IGS haplotype variants in obligate parthenogens over time. Mean intraisolate genetic and nucleotide diversities were found to differ between cyclic and obligate isolates. However, mean sequence divergence did not differ significantly, suggesting that the differences in genetic and nucleotide diversities are due primarily to differences in number and frequency of IGS haplotypes. The results indicate that in the rDNA IGS of 'D. pulex,' unequal chromosomal exchange as a result of meiotic recombination is not the primary mechanism driving concerted evolution.