The benthic fauna of Lake Maratoto, a eutrophic dy (dystrophic) lake in the Waikato basin, was investigated from 19/9/78 to 13/3/80. Core samples were taken at approximately fortnightly intervals from six stations of varying depths. Fifty taxa were recognised and the distribution and seasonal abundance of the 20 major ones is given together with notes on their biology. A major emphasis was placed on Chironomidae which, after oligochaetes, were the most numerous benthic macroinvertebrates encountered. The fauna had a clumped distribution and was concentrated on the edge of the lake. This was due to higher food supplies and more diverse sediment types on the edges and anoxic conditions in the centre. Some re-distribution of the fauna occurred during periods of wind turbulence but planktonic activity in the Chironomidae was of short duration. The annual mean standing crop of chironomid larvae was 2970 per sq. m. This was made up of 40% Calopsectra funebris, 37% Chironomus zealandicus, 5% Kiefferulus opalensis, 5% Tanypodinae, and 1% Podonominae and Orthocladiinae. Photographs of some of the distinguishing features of the chironomid taxa identified are given as well as measurements for separating larval instars. Recruitment in the chironomids was continuous but with periods of increased rates. Species with dissolved haemoglobin in the blood (e.g. Chironomus zealandicus and Cladopelma curtivalva) increased in numbers during the summer, while in winter, species which lacked this trait e.g. Calopsectra funebris were more abundant. Lowest numbers were recorded in the spring. It is hypothesised that increases in larval numbers are linked to algal production and eventual sedimentation of autochthonous organic matter. Principal component analyses were used to predict the limnological characteristics of a lake from knowledge of its surficial sediment chironomid remains. Dystrophic lakes were characterised by high numbers of most species, particularly Kiefferulus opalensis, Tanypodinae, Calopsectra spp. and Paucispini gera spp. but low numbers of Paratanytarsus a gameta. Clear lakes had proportionally more Corynocera sp. and Cladopelma curtivalva while lakes which were dominated by Paratanytarsus agameta, Orthocladiinae and Polypedilum spp. were productive and/or turbid. The developmental history of Lake Maratoto . since its formation 17,000 years ago was studied by analysis of chironomid remains from a 4 m sediment core. Numbers of fossils were low initially but climatic improvement about 14,000 yr B.P. led to an increase which culminated at 5,000 yr B.P •. Peat, which began to develop close to the lake about 12,000 yr B.P. had significant effects in the top section of the core, with maximum influence on the fauna about 7,000 years ago. Other changes in the core were largely expressions of climatically controlled fluctuations of water level which altered the area of the littoral zone. Twelve stratigraphic zones of chironomid microfossils were derived from cluster and multiple discriminant analyses. These are discussed in relation to zones derived from pollen, stratigraphy and chemical analyses of the sediments