Isozyme analysis of the rare Oenothera wolfii (Wolf's evening primrose) and the garden escape, O. glazioviana, indicates that hybridization between these species may be more widespread than morphological evidence indicates. Although both species contained low amounts of genetic variation, unique alleles were identified in both taxa. Analysis of 22 populations, including pure populations of each species, identified eight populations as containing putative hybrid individuals. Four of these putative hybrid populations were considered pure O. wolfii based on morphological analysis. This study confirms that the native O. wolfii may be at risk not only from habitat destruction, but potentially from genetic swamping where it co-occurs with O. glazioviana. These results can be used as baseline information for future genetic monitoring efforts.