The genus Melocactus has c.35 species distributed in Central and South America, the centre of diversity of the genus being the Brazilian state of Bahia, where 14 species occur. Here 11 are endemic, 5 of which are classified as Critically Endangered. In this work chromosome counts are recorded for 11 species: Melocactus bahiensis (subsp. amethystinus), M. conoideus, M. ernestii (subsp. ernestii, subsp. ernestii f. erythracanthus and subsp. ernestii f. azulensis), M. oreas (subsp. cremnophilus and subsp. oreas), M. concinnus, M. glaucescens, M. salvadorensis, M. violaceus (subsp. margaritaceus), M. zehntneri, M. xalbicephalus (a naturally occurring hybrid between M. ernestii subsp. ernestii and M. glaucescens) and M. paucispinus. M. violaceus subsp. margaritaceus presented 2n = 22, M. ernestii and M. zehntneri both had cytotypes with 2n = 22 and 2n = 44, and the remaining species had 2n = 44. The interspecific hybridization observed in the genus is discussed in relation to its reproductive biology and cytogenetic data.