-The chromosomal numbers of three species of Bidens were determined based on an analysis of somatic metaphases. The species studied were Bidens pilosa L., Bidens subalternans DC and Bidens alba (L.) DC, all of which belong to the Bidens pilosa species complex. The three species have a similar morphology: Bidens pilosa and B. subalternans are widely distributed in agricultural areas, in disturbed habitats and along roadsides, whereas B. alba occurs only along the seacoast. Bidens pilosa has 2n=72 and B. subalternans and B. alba has 2n=48. These numbers agreed with the basic chromosomal number of n=12 reported for the tribe Heliantheae and show that the number of metaphase chromosomes can be used to distinguish Bidens pilosa from the other two species of the B. pilosa complex in southeastern Brazil.