The aim of this thesis is to extend our knowledge of the New Zealand Ephemeroptera (mayflies) with particular reference to the latitudinal and altitudinal distribution of species and community assemblages. The study was centred on a nationwide survey of 81 streams located in predominantly unmodified landscapes, many of which were native forest. Three sites on each stream were sampled by light trapping of imagos (adults) and subimagos as well as supplementary collecting of nymphal stages. An initial study of four types of fluorescent lights showed that although mayflies were attracted to all four types, blacklight and blacklight-blue light were most effective. Because, blacklight-blue light was also less effective at attracting non-target terrestrial species, it was used in the nationwide survey. A total of 47 described mayfly species were obtained in the survey, 85% of the fauna known to occur in the country. Five probable undescribed species were also collected, and several previously described species were recognised as possible ‘morphospecies’ complexes, that may contain cryptic species. Of the almost 14,000 individuals taken in light traps, 39% were imagos and 61% subimagos; 63% of imagos were female, whereas the sex ratio of subimagos was approximately even (54% male, 46% female). Three gynandromorph adults were also collected. Alpha diversity throughout the country ranged from 3–24 species per stream. Species richness declined from north to south, and with increasing altitude, and the similarity of mayfly assemblages to each other declined with distance. Thirty-eight species are recognised from the North Island (15 North Island endemics), 41 from the South Island (16 South Island endemics) and 23 shared by both islands. Species richness on Stewart Island was 14 (1 possible endemic) but further study of the island’s fauna is required. Effects of latitude and altitude were revealed in the widespread and common species Coloburiscus humeralis, which increased in size (forewing length) from north to south, and with altitude. Investigations focusing on two poorly known species, Siphlaenigma janae and Isothraulus abditus, showed they had restricted and fragmented distributions, and were in need of habitat protection to reduce the likelihood of further decline. Genetic structure of S. janae, assessed using the barcoding fragment of the mitochondrial COI gene, indicated the presence of geographically distinct haplotype networks in the northern and central North Island, and the northwest South Island. Isothraulus abditus, which was found only in the North Island from Northland to Whanganui, exhibited less genetic variability (COI gene) than S. janae, but also had geographically isolated haplotypes, suggesting limited dispersal. Both species had their greatest genetic diversity in Northland, which other workers have suggested was a likely refuge from Pleistocene glaciations and subsequently a source of recolonisation further south. Overall the present study has expanded knowledge of the New Zealand Ephemeroptera by documenting patterns of species richness and community similarity in relation to latitude and altitude within the country, and provided new information and a better understanding of the distribution, ecology and conservation needs of poorly known species.