The influence of Heterocope septentrionalis, a predacious calanoid copepod, on five species of arctic pond zooplankton is investigated. Prey species coexisting with Heterocope are relatively invulnerable to predation, but prey species found in Heterocope-free ponds are very susceptible to predation. Increasing hunger level of the predator resulted in higher Heterocope feeding rates on Daphnia pulex. In experiments at 5°, 10°, and 15°C Heterocope fed at a greater rate on cladoceran prey at higher temperatures, but there was no effect of temperature for copepod prey. Visual observations showed that D. pulex is more vulnerable to Heterocope predation than Daphnia middendorffiana because of its smaller adult size and its inability to escape the grasp of Heterocope. A field experiment suggested that D. pulex would probably be excluded from ponds containing Heterocope, but D. middendorffiana could increase its population density even in the presence of the predator.