The somatic chromosome numbers of 67 hybrid tuberous begonia cultivars (Begonia x tuberhybrida Voss) have been determined. The material concerned was collected in Belgium and England and represents several distinct groups, comprising both cultivars originated at the turn of this century and those introduced recently. The old ‘Pendula’ group is characterized by the diploid chromosome number 28. Among the ‘Multiflora’ group diploid, triploid and tetraploid cultivars are represented almost equally, while the investigated ‘Multiflora maxima’ and large-flowered begonias proved to be tetraploid except for one triploid. Mitotic instability, to the extent reported by some authors, was not observed. The variation of the diploid number between 27 and 28, the triploid number between 41 and 42 and of the tetraploid number between 52 and 56 can be explained as a consequence of the fact that in the original hybridization wild tuberous species with 2n=26 and 2n=28 were involved.