A molecular approach was used to test hypotheses about the phylogeny of selected brachyuran crabs. Partial 18S ribosomal (r) RNA and rDNA sequences, ranging in length from 950 to 1,779 nucleotides, were obtained for 12 decapod crustaceans, 10 of which are generally considered to be brachyurans. Pairwise sequence-divergence estimates ranged from 0.5% to 3.9% among the so-called "true" crabs (i.e., excluding dromiids) and from 1.1% to 15.5% among the infraorders that were studied. The smallest infraorder divergence occurred between an anomuran and a dromiid crab, which suggested a dromiid-anomuran rather than a dromiid-brachyuran relationship. Results from sequence-divergence estimates and phylogenies inferred by maximum parsimony analyses of aligned sequences suggest that (1) the Raninidae demarcate the lower limit of the Brachyura and form a distinct lineage that diverged early from the lineage leading to other members of this infraorder, as indicated by a number of autapomorphic characters in the 18S rRNA molecule; (2) the Dromiidae should be removed from the Brachyura; (3) there is little support for either the family Dromiidae or the section Oxystomata as a monophyletic group; (4) there is an average substitution rate of roughly l%/50 million years, which results in insuf? ficient sequence variation within the 18S molecule to resolve relationships among brachyurans that share a post-Cretaceous divergence; and (5) the 18S ribosome-based brachyuran phylogeny is concordant with findings on spermatozoan ultrastructure and on larval rather than adult morphology. (Brachyura; phylogeny; ribosomal RNA (rRNA); ribosomal DNA (rDNA); molecular systematics.)