1. After Chaoborus flavicans became established in 1974, the species composition of zooplankton in Lake Lenore changed. Diaptomus nevadensis disappeared and densities of Diaptomus siciliswere reduced.

2. A transplant experiment showed the D. nevadensis could survive in Lake Lenore water in the absence of Chaoborus. The disappearance of D. nevadensis was probably a consequence of Chaoborus predation.

3. Although densities of Daphnia pulicaria† were similar before and after the colonization of C. flavicans, juvenile D. pulicaria began producing neck-teeth during the first summer Chaoborus was present.

4. The seasonal pattern of neck-teeth production by juvenile D. pilicaria did not correlate with density of Chaoborus but rather with a combination of water temperature and Chaoborus density, suggesting that metabolic processes are a component of induction of neck-teeth in this species.