Polyploidy is the most common chromosomal mechanism involved in the evolution of plants. However, the emergence of polyploid individuals does not guarantee the establishment of a new lineage, and the relationship between polyploidy and reproductive biology is therefore relevant. Libidibia ferrea is a legume tree that has diploid and tetraploid populations. In this work we analyse the reproductive biology of the species to verify the degree of reproductive isolation between the two cytotypes. Obser-vations on phenology, floral morphology, biology, and visitors, breeding system, reproductive success and repro-ductive isolation were made for both cytotypes in two municipalities of northeastern Brazil. Cytotypes differed for all morphometric parameters analysed, with tetraploids exhibiting higher mean values than diploids. Both cyto-types had the same effective pollinators (native bees from the genera Centris and Xylocopa, and the introduced Apis mellifera). However, since stamens of diploids and tet-raploids were of different size, it is possible that spatial separation occurs when pollen of each cytotype is depos-ited on a bee's body. Diploids were self-incompatible (ISI = 0) and exhibited high fruit-set after intraploidy crossing (20 %), whereas tetraploids were self-compatible (ISI = 0.47) and set only 3.9 % of fruits after intraploidy crossing. Both cytotypes showed low fruit-and seed-set under natural conditions, and there was no fruit-set after crosses between them. Data are discussed in relation to the establishment of polyploids, to the pattern of distribution of the species and to the taxonomic implications. The cyto-types of L. ferrea should be considered as distinct species, since they are reproductively isolated from each other.