Caesalpinia L. (Leguminosae: Caesalpinioideae) is a genus of some 150 species of spiny or unarmed trees, shrubs, and climbers of the Old World and New World tropics and subtropics. About 40 native species and several introduced species are reported for the West Indies, and about 30 species are cited as endemic to a single island or archipelago. Caesalpinia monensis Britton is restricted to two small populations on Mona Island, Puerto Rico; the species has been under study for several years by Gary J. Breckon and the first author in collaboration with the U.S. Fish and Wildlife Service, Caribbean Field Office. To investigate its genetic diversity and possible relationships in the Greater Antilles, the technique of ISSR (Inter-Simple Sequence Repeat) markers was applied to population samples of leaf material of C. monensis as well as samples of C. bonduc (L.) Roxb. (from Mona Island), C. buchii Urb. and C. domingensis Urb. (from the Dominican Republic), and C. pauciflora (Griseb.) C.Wright (from the Dominican Republic and Cuba). Following a preliminary screening, four primers were selected and yielded a total of 47 DNA fragment bands among a total of 33 plant samples. UPGMA cluster analysis, neighbor-joining, and principal coordinates ordination using a DICE similarity matrix from band presence/absence data demonstrate that C. monensis exhibits some genetic variability; the two populations intermingle, sharing many ISSR bands, suggesting either active gene flow between them or the retention of historic polymorphisms from a continuous past distribution. Caesalpinia monensis is very close to C. domingensis, and the two may not represent distinct species. Caesalpinia pauciflora samples from the Dominican Republic and Cuba are more or less well separated, and this species shows an affinity to C. buchii. The scrambling, widespread C. bonduc is distinct from the other species, which are erect Greater Antillean endemics.

Key words: Caesalpinia monensis, endemic, genetic diversity, Greater Antilles, ISSRs, Leguminosae