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In the Toolik Lake region of arctic Alaska, ponds contain one of two distinct zooplankton communities. Small ponds are typically dominated by Daphnia pulex and medium and large ponds are typically dominated by the larger daphnid, Daphnia middendorffiana. An invertebrate predator, Heterocope septentrionalis, exists in the medium and large ponds with D. middendorffiana. D. pulex were exposed to Heterocope kairomone in laboratory and in situ experiments in an effort to explain why these two species do not coexist in nature. An increase in average egg number per brood was observed when D. pulex were exposed to predator kairomone both in laboratory and in situ studies. In laboratory studies, D. pulex increased size at 24 hours old in response to Heterocope kairomone. Behavioral changes, such as an alternation in vertical position, were not observed when D. pulex and D. middendorffiana were exposed to predator kairomones