Isozyme markers were used to identify several cultivars of purple loosestrife (Lythrum spp.) and interspecific hybrids. There were three zones of activity for phosphoglucomutase (PGM) and phosphoglucoisomerase (PGI) and two zones for malate dehydrogenase (MDH) in purple loosestrife cultivars. Allelic constitution could not be characterized due to the polyploid nature of purple loosestrife and the possibility of intergenic dimerization. Coefficients of genetic similarity were used to estimate the degree of relationship between purple loosestrife cultivars. Cluster analysis indicated that seven cultivars originating from L. salicaria L. were not distinguishable from eight cultivars originating from L. virgatum L., indicating possible limitations of isozyme analysis for cultivar differentiation based on species origin. All but two cultivars (`Morden Gleam' and `Morden Rose') could be distinguished from one another by isozyme phenotype. This result suggests that isozymes may be useful for cultivar fingerprinting if additional isozyme systems could be resolved. `Robert' appeared morphologically heterogeneous, and plants could be differentiated based on isozyme banding patterns. Also, two putative clones of `Stichflamme' (one marketed under its English synonym `Fire Candle') possessed distinct isozyme phenotypes, indicating a lack of clonal integrity.