Zooplankton populations were studied in three hydroelectric reservoirs on the Waitaki River, South Island, New Zealand (Lakes Benmore, Aviemore and Waitaki) both by sampling directly from the reservoirs (using vertical net hauls) and by monitoring the discharges with automatic plankton samplers. Temperature,food supplies and retention times were major factors influencing seasonal cycles in the most upstream reservoir, Lake Benmore, while populations in the two downstream reservoirs were also strongly influenced by discharges of plankton from Lake Benmore. vertical distributions of zooplankton species in the three reservoirs were investigated by sampling with a Van Darn water bottle. Adults, nauplii and copepodites of the copepod Boeckella dilatata generally showed some spatial separation during the day and B. dilatata, the cladoceran, Bosmina meridionalis, and the rotifer, Asplanchna priodonta, migrated to the surface at night. A more detailed investigation was made of the population dynamics of Boeckella dilatata. Throughout 18 months of sampling, no clear temporal sequences of developmental stages were identified in discharges from Lake Waitaki. However, there were clear indications that population structure in Lake Waitaki was affected by discharges of B. dilatata from Lake Benmore. Egg development rates of B. dilatata were investigated in the laboratory and were used to calculate the finite birth rate (B) for the days of sampling. Daily production was estimated from standing crop biomass and turnover times (calculated as 1/B). Mean clutch sizes and finite per capita birth rates of B. dilatata in the two arms of Lake Benmore were markedly higher than values recorded for populations of calanoid copepods (including B. dilatata) in other New Zealand lakes. The feeding behaviour of B. dilatata was investigated by exposing the animals to a suspension of micronic plastic beads. The maximum bead size ingested by B. dilatata copepodites increased in the later instars and the largest bead ingested by adult females was approximately 40 μm.