Inverted repeats have been found to occur in both prokaryotic and eukaryotic genomes. Usually they are short and some have important functions in various biological processes. However, long inverted repeats are rare and can cause genome instability. Analyses of C. elegans genome identified long, nearly-perfect inverted repeat sequences involving both divergently and convergently oriented homologous gene pairs and complete intergenic sequences. Comparisons with the orthologous regions from the genomes of C. briggsaeand C. remanei show that the inverted repeat structures are often far more conserved than the sequences. This observation implies that there is an active mechanism for maintaining the inverted repeat nature of the sequences.