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This study is based on about 7,000 specimens of earthworms, collected from the mainland of New Zealand and its outlying islands, Kermadec Is., Snares Is., Campbell Is., Auckland Is., Antipodes Is., Macquarie Id., and Chatham Is. Previous work on New Zealand earthworms is reviewed, and where necessary revised. 192 species of earthworms, belonging to 34 genera, are recognised. Of these, 178 species, of 27 genera, are referred to the family Megascolecidae, and 14 species, of 7 genera, to the family Lumbricidae. Diagnoses and keys are given for all species, genera, tribes, subfamilies and families within the fauna. To aid in the identification of species and to enable comparisons between species to be made, the principal morphological characters of the species of all genera with two or more New Zealand species have been tabulated. The classification of Megascolecidae is reviewed and two subfamilies, Acanthodrilinae and Megascolecinae, are recognised, in place of the four subfamilies formerly recognised. The general morphology of Megascolecidae is described and figured and the taxonomic characters of genera and species of Megascolecidae and Lumbricidae are discussed. Wherever possible, the current diagnoses of megascolecid genera have been amplified, and where necessary modified. Four new genera and 53 new species of Megascolecidae are described. The geographical distribution of megascolecid species is mapped, and the taxonomic relationships of the genera and species are discussed. The mainland of New Zealand is divided into nine major areas, based on the distribution of Megascolecidae. The fauna of each area and of outlying islands is listed and the relationships of groups of species within individual areas and in adjacent areas and groups of islands are discussed. Differences between the areas are explained in terms of current ideas of the geological, climatic and vegetational history of the New Zealand region. Evidence for geographical speciation is discussed. Various theories of the origin of New Zealand Megascolecidae are discussed and it is concluded that on the evidence of known distribution it is most likely that New Zealand earthworms came originally from the IndoMalayan or Australian region and entered New Zealand across a land connection from the north. Megascolecinae may have arrived during Tertiary times and Acanthodrilinae during the Cretaceous period. It is concluded that the earthworm faunas of New Zealand subantarctic islands are of recent origin and that the faunas of Campbell and Antipodes Is. have been derived very recently from the fauna of Auckland Is. The environmental conditions necessary for the survival of earthworms are defined and the habitats in which New Zealand species have been found are classified and their characteristics examined in the light of the conditions listed as necessary. The distribution and ecology of New Zealand earthworms are examined first in relation to the distribution and characteristics of the major New Zealand soil groups defined by Taylor (1948) and secondly in relation to each of the five environmental factors considered by Taylor to comprise the significant causal agents of soil formation. Evidence for ecological speciation is discussed.