In the present study, five species of Contracaecum Railliet and Henry 1912 (Nematoda: Ascaridida), including Contracaecum bancrofti, Contracaecum microcephalum, Contracaecum variegatum, Contracaecum eudyptulae, and Contracaecum ogmorhini, were redescribed using light and scanning electron microscopy. In addition, in order to elucidate their taxonomic status, first and second internal transcribed spacers (ITS-1 and ITS-2, respectively) of nuclear ribosomal DNA of each morphospecies were characterized. Analyses of sequence and morphological data sets suggested that C. bancrofti, infecting the Australian pelican Pelecanus conspicillatus, is a valid species and is distinct from C. micropapillatum reported from pelicans in the northern hemisphere. C. microcephalum from cormorants Phalacrocorax melanoleucos and C. variegatum from the darter Anhinga melanogaster and from P. conspicillatus as well as C. eudyptulae from the little penguin Eudyptula minor were also considered as distinct species, which can be differentiated from one another morphologically based on the lengths of spicules and genetically based on the sequences of ITS-1 and ITS-2. Comparison of sequence data of ITS-1 and ITS-2 for the members of C. ogmorhini sensu lato from pinnipeds with those of previous studies suggested that only ITS-2 can be used for differentiation between C. ogmorhini sensu stricto and Contracaecum margolisi, occurring in the southern and northern hemispheres, respectively. Analyses of the ITS-1 and ITS-2 sequence data of Contracaecum spp. in the present study supported the distinction among species of Contracaecum based on morphological data and were useful in confirming the taxonomic status of individual species in Australia.