The genomes of two parasitoid wasps, Diadromus pulchellus and Eupolmus vultleti, and the honey bee, Aphis mellifera, contain few interspersed repeated sequences corresponding to transposons (Tn). This suggests that the genomic organisation of Hymenoptera could be due to the elimination of deleterious Tn in haploid males. We have used restriction-fragment length polymorphism analysis to show that nondelectrious Tn are present in the DNA (rDNA) encoding ribosomal RNA of twelve species of Hymenoptera. Sequence analysis of the 28S rDNA type-I and type-11 insertion-rich regions of 80 species showed that this region is very highly conserved ((95.8%). A consensus sequence and restriction map of the RDNA region were established. none