A taxonomic re-examination of the Posidonia ostenfeldii complex was undertaken based on field collections in Western Australia. Broad and narrow leaves of seagrass of the Posidonia complex from Success Bank were compared to P. kirkmanii, P. coriacea, P. robertsoniae and P. denhartogii, identified based on their distribution as described by Kuo and Cambridge (1984). The broad and narrow leaf varieties of Posidonia on Success Bank were unable to be identified to species due to the overlap of character ranges between groups. Multivariate analysis of 10 vegetative characters examined indicated a continuous variation of character traits within the complex, suggesting the existence of a morphological continuum. Genetic variation was also examined using starch gel electrophoresis, restricted to Success Bank and Marmion populations. No allozyme polymorphism was observed among all the samples tested across both populations and varying leaf widths. All loci were monomorphic and homozygous. This study implies that the key of vegetative morphological characters, upon which five species of the P. ostenfeldii complex was erected, was not effective for the identification to species of multiple samples from any one location. Presently there is no justification for supporting Posidonia coriacea and Posidonia robertsoniae as separate species and it is further recommended that the whole group be reanalysed. (C)2000 Elsevier Science B.V. All rights reserved.