In a small, shallow beaver pond near Georgian Bay, Ontario, life histories of each of the three limnetic copepods found were basically similar in the 3 yr 1970–72 although the patterns were occasionally obscured. Cyclops bicuspidatus thomasi presumably emerged from diapause at stage C 4 when the ice broke and went through one complete cycle to C 4 in 3–4 wk, reentering diapause in late spring. Mesocyclops edax emerged from diapause at C 4 slightly later in spring and underwent four complete cycles, reentering diapause in midautumn; normal time to complete a generation was about 1 mo. Diaptomus reighardi hatched from resting eggs in early May and produced four complete generations, the last maturing in early autumn; development of spring and summer generations required 1 mo and that of autumn generations about [Formula: see text] mo. During 1972 lowered water levels resulted in increased heat content of the pond per unit volume and this was reflected in some acceleration of development rates of M. edax and D. reighardi. At most sampling times M. edax had about double the fecundity of D. reighardi indicating a correspondingly high mortality rate that might be the result of lower egg viability, cannibalism, or competition between nauplii of the two species.