A 656-pb fragment of the mitochondrial cytochrome b gene was sequenced for six species of Central and South American potoos (genus Nyctibius, Nyctibiidae) as well as for selected representatives of all other caprimulgiform families. Sequence divergence among potoos was much higher (11.1-16.2%) than has typically been observed among congeneric species of birds, suggesting that the species of Nyctibius are quite old. Divergence among families was also quite high (13.7-21.8%), confirming recent DNA-DNA hybridization studies. Such high genetic divergences in a functionally constrained gene like cyt b indicate that many of those sites which are free to vary will have experienced multiple substitutions. We therefore performed phylogenetic analysis using parsimony under a variety of weighting schemes intended to reduce the effect of multiple substitutions. The monophyly of all the traditional caprimulgiform families was confirmed and a number of new hypotheses of relationship emerged. From our analysis, it appears that the oilbird (Steatornis) is an ancient member of the order, and it is not closely related to the potoos. We also note the close link between Aegothelidae and Caprimulgidae, and the basal position of the Podargidae/Batrachostomidae clade in the phylogeny. These results are in accordance with several classical works of the first half of the century. The relationships of the various Nyctibius species to each other have not been fully resolved; however, a close link between N. leucopterus and N. maculosus appears to be highly probable from our data.