Cross-compatibility within and among populations of the perennialEriastrum densifolium was investigated to determine if sterility barriers have evolved within this polytypic species, and to correlate any such barriers to previously known patterns of genetic and morphological variation. Furthermore, the reproductive affinities of the endangeredE. densifolium subsp.sanctorum are investigated. Results indicate that no single population or group of populations is reproductively isolated from all other populations, and that F1 hybrids are fully fertile. Interspecific crosses (withE. eremicum andE. sapphirinum) were largely unsuccessful. The endangeredE. densifolium subsp.sanctorum demonstrates high interfertility with all other populations.