New information on the vocalizations and behavior of populations of the Rufous Cacholote (Pseudoseisura cristata), combined with a morphometric analysis of museum specimens, reveal that it actually consists of two biological species: a form that is restricted to the arid caatinga region of northeastern Brazil (P. cristata), and a wider-ranging form (P. unirufa) that occupies seasonally flooded savannas and deciduous woodlands in northern and eastern Bolivia, northern Paraguay, and southwestern Brazil. Long considered a subspecies of P. cristata, P. unirufa is shown to be morphologically, vocally, and ecologically distinct from allopatrically distributed populations of P. cristata. Reciprocal playback experiments of tape-recorded vocalizations indicate that neither form responds to the other's vocalizations. The two species may also differ in their breeding systems and social structures. P. cristata is shown to be a cooperative breeder, with nonbreeding helpers assisting in territorial defense, nest-building, and care of nestlings. No evidence of cooperative breeding was found in P. unirufa.