Ptilotus (Amaranthaceae) is an Australian genus with over 100 species, most of which occur in arid Western Australia. Ptilotus has been a taxonomically difficult genus; despite rigorous morphological studies into the genus over many years, previous workers have found it difficult to delimit infrageneric groups due to inconsistent morphological variation. With the goal to establish a phylogenetic framework for the genus, 100 taxa were sampled, including 87 Ptilotus spp., and the ITS nrDNA and matK cpDNA were sequenced. The phylogeny was reconstructed using Bayesian, maximum likelihood and maximum parsimony analyses on separate and concatenated datasets. Morphological characters were assessed and compared to clades on the phylogeny to identify synapomorphies and aid in the construction of an infrageneric classification. A diversification rate analysis was used to identify rate shifts in speciation across the phylogeny. Four major clades of the monophyletic Ptilotus were resolved, three small clades together comprising 27% of sampled taxa and a large, diverse clade comprising the remaining 73%. Four floral synapomorphies were identified as uniquely occurring within the latter, although none were common to all taxa in the clade. The diversification rate analysis identified a probable rate shift at the base of Ptilotus, indicating that the genus may have undergone a rapid diversification early in its evolution. This rapid diversification provides a plausible explanation for the lack of consistent variation in morphology among the major clades. [ABSTRACT FROM AUTHOR]