The limits of American genera of Myrtaceae have historically been difficult to define. One of these being Mosiera, a group of about twelve species of trees and shrubs that occur in tropical dry, deciduous or partially deciduous forests, mainly in the Caribbean, with much of the diversity in Cuba. Two species occur in eastern Mexico, one of these also occurring in Guatemala, and one species occurs in South America. Since its description by Small in 1933, when it included only two species from southern Florida, the definition of the genus has been controversial and unstable. Mosiera has a combination of characters which lend it to being mistakenly placed into other superficially similar genera (i.e., Eugenia of the subtribe Eugeniinae, or Myrtus or Psidium of the subtribe Myrtinae). While Moisera can be separated from Eugenia by embryo structure, the delineation of Mosiera from other genera in the Myrtinae was not clarified until recently. Landrum and Sharp (1989) used seed coat characters in combination with floral characters to distinguish Psidium from Mosiera and other related genera. This study expands the work of Landrum and Sharp (1989) to include additional taxa. Compared to other species of Mosiera which have smooth integuments, M. ehrenbergii and M. prismatica have a raised net-like secondary sculpture and a papillose surface, respectively. Preliminary sequence data from the ITS region of nrDNA supports the distinctness of Mosiera and the taxonomic utility of seed coat characters in the American Myrtinae.