A detailed study of mitosis and meiosis in Pyrus species has revealed that all species are diploid, with 2n = 34, though several cultivars and variants are triploid. All diploids had regular 17 II. In triploids, trivalents represented the maximum association, and no cell had more than a total of 17 bivalents and trivalents combined. These data do not support the hypothesis of autosyndesis within the third set of 17 chromosomes. Alternatively, the data support the Sax (1931) concept that the Pomoideae group arose as an allopolyploid between two primary forms having basic numbers of 8 and 9, respectively. Speciation within the genus Pyrus, as in other genera of the Pomoideae, has occurred without a change in chromosome number.