Phylogenetic analyses of data from the plastid genes rbcL and matK were conducted using parsimony and maximum likelihood methods to assess the monophyly of the genus Myriophyllum (Haloragaceae) and the relationships among the species. Myriophyllum is an aquatic genus that consists of approximately 40 species with the greatest species diversity centered in Australia. Relationships among many aquatic plants have been notoriously difficult to assess due to their often convergent and highly plastic vegetative morphology and structurally reduced flowers. All of these factors have made it difficult to determine species limits and relationships among Myriophyllum. MatK and rbcL have been highly informative in delimiting relationships among species of Myriophyllum. Our results indicate two well supported clades in the genus. One clade consists predominantly of plants with eight stamen flowers, the other predominately four stamen flowers. A clade of North American endemics (Schindlerís Subgenus Tessaronia) is also well supported. Currently, the cpDNA data support a monophyletic Myriophyllum; although more inclusive sampling among all genera in the family will be needed. Recently acquired material of Haloragaceae genera and additional species of Myriophyllum from Australia are currently being processed for inclusion in expanded analyses.

Key words: aquatic plants, cpDNA, morphology, Myriophyllum, Phylogenetics