Cytophotometric estimation of 4C DNA content, karyotype analysis and Interphase Nuclear Volume (INV) were carried out in nine species of Ferocactus of the family Cactaceae. Significant interspecific variations in nuclear DNA amount were noted with a constant somatic chromosome number (2n=22). The 4C DNA content varied from 7.66 pg in F. emoryi to 9.74 pg in F. gracilis. The INV varied from 390.36 μm³ in F. pilosus to 564.32 μm³ in F. gracilis. The average chromosome length and volume varied from 2.66 μm in F. emoryi to 2.96 μm in F. gracilis and 1.92 μm³ in F. histrix and 2.77 μm³ in F. gracilis. The mean 4C DNA content showed a significant positive correlation with chromosome length, volume and INV. The structural alterations in the chromosomes as well as loss or addition of highly repetitive sequences in the genome suggest variations in the nuclear DNA at interspecific level during macro-and micro-evolution of the species.