Phylogenetic analyses using parsimony and maximum likelihood methods were conducted with data from the plastid genome (rbcL, matK and trnK introns) to assess relationships among Haloragaceae genera and among species in the aquatic genus Myriophyllum. The Haloragaceae consist of 8 genera, three of them aquatic (Proserpinaca, Laurembergia, and Myriophyllum). Myriophyllum (55 species) is the largest genus in the family and has its greatest species diversity centered in Australia. Relationships among many aquatic plants have been notoriously difficult to assess due to their convergent and highly plastic vegetative morphology and reduced flower size. All of these factors have made it difficult to determine species limits and relationships among Myriophyllum or to assess its relationship to other genera in the family. RbcL, matK and the trnK introns have been very informative in delimiting relationships among Haloragaceae genera and among species of Myriophyllum. Results indicate an Australian origin for the family with possible multiple origins of the aquatic habit. Results also suggest a monophyletic Myriophyllum consisting of two well supported clades. A clade of North American endemics (Schindlerís Subgenus Tessaronia) is well supported and has its closest sister taxa in Australia.

Key words: Haloragaceae, matK, maximum likelihood, Myriophyllum, parsimony, trnK introns