Karyotype studies including determination of somatic chromosome number, total chromosome length and volume, 4C DNA content and INV were carried out in 6 species of Melocactusof the family Cactaceae. Somatic chromosome numbers 2n=22 in M. albicephalus, M douradaensis and M lensselinkianus and 2n=44 in M borhidii, M bredorooanus and M. ernestii were recorded for the first time. Significant interspecific variations in nuclear DNA amount were noted. The 4C DNA content varied in the diploid species from 21.95 pg in M. douradaensis to 24.31 pg in M lensselinkianus and in the polyploid species from 35.76 pg. M ernestii to 39.21 pg in M borhidii. The correlation coefficient studied showed that the chromosome volume, nuclear DNA content and INVwere positively correlated. No interdependency was found between average 4C DNA content and average chromosome length. The structural alterations in the chromosomes as well as less or addition of highly repetitive sequences in the genome showed variations in nuclear DNA at interspecific level indicating a macro-change of nuclear material during evolution.