Seven species of the genusEucalyptuswere studied cytogenetically (E.deanei, E. dunni, E. grandis, E. maculata E. propinqua, E. saligna and E. tereticornis).The species showed a symmetrical karyotype with 2n=22 chromosomes,with chromosome length ranging from 0.58 m to 1.39 m. Karyotypic analysis indicated homogeneity of morphology and of chromosome number for most of the species of this genus studied here, although casual disploid species with 2n=24 have been found in previous studies. According to these data, a basic number of x=11 was established for this genus. The evolutionary tendency probably occurred by structural alterations (deletions, duplications, additions and translocations) and in some cases by aneuploid chromosome alterations.