Clethra alexandri Griseb. is a shrub or low tree endemic to high montane forests of the Blue Mountains, St. Andrews and St. Thomas Parishes, Jamaica. Material in flower and bud was collected at Blue Mountain Peak, St. Thomas Parish (i.e., Judd 659 [A]), and buds of various sizes were fixed in modified Carnoy’s solution (4 parts chloroform, 3 parts absolute alcohol). Young stamens were squashed and stained in 1% acetocarmine and examined using a light microscope, resulting in five countable cells in diakinensis or metaphase I. All clearly showed eight bivalents, indicating a diploid number of 16 for this species (Figure 1). Clethra alexandri is a member of section Cuellaria (Sleumer 1967, Thomas 1961), a clade (Fior et al. 2003) including ca. 39 species, which is widely distributed in the Neotropics, and characterized by winged seeds and flowers with an included, non-trifid style. In contrast, the members of the Asiantemperate North American-Macaronesian section Clethra characteristically have unwinged seeds and exserted, usually trifid styles. Both sections share a basic chromosome number of x58 (Schneider and Bayer 2004, Tanaka and Oginuma 1980), as seen in the chromosome counts for C. arborea Ait. (n58; sect. Clethra, as circumscribed by Fior et al. 2003), C. alnifolia L. (n516, a tetraploid; sect. Clethra), C. lanata Martens & Gal. (n58; sect. Cuellaria), and C. pringlei S. Wats. (n58; sect. Cuellaria) (Hagerup 1928, Kyhos 1965, Reed 2005). The highest reported number for the group is in Clethra barbinervis Sieb. & Zucc. (of sect. Clethra), which is a decaploid (i.e., n540) (Tanaka and Oginuma 1980). This report increases the evidence for x58 as the basic number in Clethra, and it is noteworthy that all species of sect. Cuellaria for which chromosome counts are known are diploid (2n516). Funds for field work in Jamaica were provided by a National Science Foundation Grant (BMS 75-21656) and by the Anderson Fund, the Arnold Arboretum, and the Atkins Fund, the Gray Herbarium.