Nolina brittoniana is a perennial agave endemic to the central ridges of the Florida peninsula. Its upland scrub and sandhill habitats have suffered extensive destruction and fragmentation, and indirect alteration through fire suppression. We examined genetic variation in 48 populations from throughout its range using isozymes. Features of the life history and ecology of the species led us to predict that this federally listed plant would not be genetically impoverished, in spite of its narrow range. However, we found lower values for percentage of polymorphic loci, average numbers of alleles per locus and expected heterozygosity than those generally reported for endemic plants. Populations were fairly well differentiated (mean FST = 0.363). Inbreeding rates were low and allele number and frequency did not indicate recent bottlenecks. Significant clines in allele frequency were detected along the north-south axis of distribution. Spatial structure of genetic variation and high population differentiation reflect the patchy distribution of scrub habitats and support the need to preserve populations from throughout N. brittoniana's range.

Key words: Agavaceae, Florida, isozyme analysis, Lake Wales Ridge, Nolina brittoniana, rare plants