A study of Sax (1954) involving 59 species and cultivars of Cotoneaster showed that 43 were triploid, among them 24 species. As the number of triploids found by Sax was strikingly large, Zeilinga (1964) also carried out an investigation on the chromosome number in this genus, particularly in species and cultivars under cultivation. Among the 58 origins investigated, only 2 triploids were encountered, both cultivars. By contrast all species were diploid (2n = 34) or tetraploid (2n = 68). Complementary to Zeilinga’s investigation, chromosome counts were made in species which are generally not in cultivation. This paper reports the results.