Aspects of the biology of Paranephrops planifrons White in Lake Rotoiti were studied and comparisons made with P. planifrons inhabiting neighbouring Lakes Okataina and Rotorua. Two migrations upwards occurred, in spring and summer, the first thought to be associated with temperature and egg-hatching, the second due to hypolimnetic deoxygenation. Hence in late summer and early autumn crayfish were concentrated above and within the thermocline. Movement downwards occurred in early winter, after lake turnover and this was thought to be associated with gonad maturation, induced by low temperatures and shorter photoperiod. During winter crayfish distributed evenly over the lake floor. There were two breeding seasons, in late autumn and winter (the larger of the two) and late spring and summer. Lake Okataina P. planifrons bred annually, during late autumn and winter, and showed no apparent seasonal movement patterns. Moulting occurred during the warmer months. Average densities of 0.030, 0.014 and 0.003 adult crayfish/m. ² were calculated for Lakes Rotoiti, Okataina and Rotorua respectively. In these populations a broad inverse relationship was found between density and mean population size, for individual sizes were small in Wright’s Bay, Rotoiti compared to those in Lakes Okataina and Rotorua and other parts of Rotoiti. Sizes of males and females were not significantly different and mean size was constant throughout all depths. Sex ratios of 1.9:1 and 2.3:1 in favour of males were found in Lakes Rotoiti and Okataina respectively. Stomach content analyses revealed that the diet was mainly detritus, but also included animal remains and vascular plant material. Sediment analyses showed that food quality was essentially the same at all depths. Feeding was continuous at depths greater than 20m. but in shallower water it took place only at night. It appeared that P. planifrons does not display homing and the home range is extensive. Observations revealed that territorial behaviour occurred and males were not dominant over females, although larger crayfish were dominant over smaller animals.