The histone genes of the midge Chironomus thummi thummi are organized in tandemly repeated gene groups, each containing the four core histone genes plus an H1 gene. These repetitive gene groups are found at five different loci, linked on one chromosomal arm. In addition to the clustered gene groups an isolated histone gene group exists which is found spatially separated on a different chromosome ("orphon" gene group). These orphon genes have been cloned and analysed in detail. Nucleotide sequence and in situ hybridization data suggest that the orphon gene group was established early during chironomid speciation, possibly by a transposition-like mechanism. This allowed the genes to be moved as an integer group. The comparison of orphon and "clustered" histone genes in C. thummi thummi indicates that the early spatial separation of the orphon genes from their tandemly organized relatives may have stimulated divergent sequence evolution. This is particularly true for the orphon H1 gene, which has diverged considerably by unusual mutation mechanisms. The translocation of normally clustered genes to new genomic sites may favour the generation of sequence variants, which could fulfill specialized functional tasks.