The karyotype was studied in six genotypes of pitahaya (Hylocereus spp.). The chromosomes were observed in cells from root tips pretreated in a watery solution saturated with p-dichlorobenzene for 5:30 to 6:00 hours and fixed in Carnoy. Chromosomes were stained in Feulgen and 1.8 % propionic orcein. Chromosomes were measured on scanned photographs of cells and processed in Image Tool, version 1.8, software. The six genotypes of pitahaya were diploid, exhibiting a chromosome number of 2n = 2x = 22. Chromosome length varied from 2.46 to 5.06 µm. The karyotypes of the pitahayas were uniform in length and arm relationships. Two cytotypes were distinguished, both with metacentric (m) and submetacentric (sm) chromosomes. Cytotype A (10 m + 1 sm) was present in five genotypes (BHA, BV22, A28, A39, and Rojo), and cytotype B (9 m + 2 sm) in only one (Solferino). The karyotypes showed two to three secondary constrictions in the short arms of metacentric chromosomes. The similar karyotype characteristics of the genotypes studied could indicate that they belong to the same species, Hylocereus undatus.

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