The discovery and taxonomic history of fluttering shearwater (Puffinus gavia (Forster) and Hutton's shearwater (Puffinus huttoni Mathews) are reviewed. Taxonomic theory, where appropriate to this thesis, is discussed. The external morphology of P. gavia and P. huttoni is compared. No single external measurement or plumage character separates more than 60% of birds examined. The best system of identification is to compare the ratio of different body parts within an individual bird. The distribution of P. gavia and P. huttoni is compared. Hutton's shearwater feeds further out to sea and it is believed to be a migrant species wintering in north west Australian waters. The fluttering shearwater is believed to be a semi-migrant species with only the juveniles spending time in south east Australia. The red cell enzymes of P. gavia, P. huttoni and P. griseus are compared. There are differences in two esterase loci between gavia and huttoni, while P. griseus is more distantly related. Nei's genetic identity values are calculated. The systematic value of electrophoretic data is discussed. The relationship of an undescribed subfossil shearwater to P. gavia and P. huttoni is discussed. An outgroup analysis to other shearwater species is carried out according to phylogenetic (cladistic) theory. The subfossil shearwater is most closely related to the fluttering shearwater, and these two form a sister group to Hutton's shearwater. These three species are a sister group of P. opisthomelas. The relationship between the many P. assimilis subspecies, the black-backed Manx shearwaters, and the gavia, huttoni and opisthomelas group was not resolved. Puffinus nativitatis is more closely related to the Manx and the little shearwaters than to the P. griseus, P. tenuirostris group.