The pathway that controls sexual fate in the nematode Caenorhabditis elegans has been well characterized at the molecular level. By identifying differences between the sex‐determination mechanisms in C. elegans and other nematode species, it should be possible to understand how complex sex‐determining pathways evolve. Towards this goal, orthologues of many of the C. elegans sex regulators have been isolated from other members of the genus Caenorhabditis. Rapid sequence evolution is observed in every case, but several of the orthologues appear to (...) have conserved sex‐determining roles. Thus extensive sequence divergence does not necessarily coincide with changes in pathway structure, although the same forces may contribute to both. This review summarizes recent findings and, with reference to results from other animals, offers explanations for why sex‐determining genes and pathways appear to be evolving rapidly. Experimental strategies that hold promise for illuminating pathway differences between nematodes are also discussed. BioEssays 25: 221–231, 2003. © 2003 Wiley Periodicals, Inc. (shrink)