Lucerne flea, Sminthurus viridis L., is an annual pest in pastures in the Auckland/Waikato area. It damages legumes, particularly white clover, from late autumn until early summer, with a peak in infestation in spring. No research into the seasonality of the lucerne flea (LF) has been done in New Zealand. It was therefore decided to study the bionomics and population dynamics of LF, and aspects related to the damage, caused by LF. The study showed that the life cycle of LF consisted of three nymphal instars, one adult male and three adult female instars with the males being smaller and lighter in weight than the females. The population of LF built up steadily from March or April onwards to reach a peak in numbers in October or November after which the numbers declined to a low level and LF were only present as diapause eggs. Rainfall in March and April played an important role in the hatching of the diapause eggs and influenced the survival of the first instar larvae. The accumulated temperature (°D) above a zero development threshold of 4 °C determined the number of generations of LF per year, with five generations in 1982 and four in 1983 in Huntly. Shutting up a paddock for hay production as well as the consecutive cutting of the hay influenced the number of LF in the field. Population dynamic studies of LF showed that LF populations had overlapping stages of development as well as overlapping generations during the study period. Examination of the data with methods, equivalent to the k-factor analysis, showed that LF populations were prone to great changes in number at the beginning of the immature stage of the life cycle of the males and the end of the life cycle of the females, but fairly stable at the time that sexual maturity was reached. The locomotory activity of LF took place during the day and night and all growth stages were active. No statistically significant relationship could be found between the locomotory activity and temperature or rainfall. The mite, Bdellodes lapidaria, a predator of LF, was found in the study sites in Huntly but their numbers were so low that no control effect on LF could be established. Feeding trials showed that adult female LF ate significantly more than adult male LF, and that the feeding rate was highest at 15 and 20 °C and a photophase of 12 and 16 hours light. Other results suggested that some feeding activity would also occur at night. Severe damage by LF in the field reduced the leaf surface area of white clover plants by up to 40 % during this study, and most severe damage was observed in October and November. Observations in the field and the laboratory established that LF is a wasteful feeder.