DNA-DNA hybridization studies of insects, more specifically Drosophila and cave crickets, have revealed interesting patterns of genome evolution that contrast markedly with what has been seen in other taxa, especially mammals and birds. Insect genomes are composed of sections of single-copy DNA with extreme variation in rates of evolutionary change. This variation is more extreme than between introns and exons; introns fall into the relatively conserved fraction of the genome. Attempts to calculate absolute rates of change in Drosophila DNA have all led to estimates some 5-10 times faster than those found in most vertebrates; this is true even for the more conservative part of the nuclear genome. Finally we point out that morphological similarity, chromosomal similarity, and/or ability to form interspecific hybrids is often associated with quite high levels of single-copy DNA divergence in insects as compared to mammals and birds.