The 36 species of Myriophyllum found in Australia are keyed, described and illustrated. Fifteen species (M. alpinum, M. austropygmaeum, M. balladoniensis, M. costatum, M. crispatum, M. decussatum, M. echinatum, M. im- plicatum, M. limnophilum, M. lophatum, M. muricatum, M. papillosum, M. petraeum, M. simulans, M. striatum), one subspecies (M. callitrichoides ssp. striatum) and two varieties (M. gracile var. lineare and var. laeve) are newly recognized, and one new com- bination (M. pedunculatum ssp. longibracteolatum) is proposed. The grouping of the Australian species into 'alliances' is discussed, and their relationships with some extra- Australian species are explored.