AbstractAn analysis of digitized three‐dimensional video records of free‐swimmingDaphnia pulexidentified six measurements of swimming behavior that can be used to quantify individual behavioral responses to pesticides or natural chemical signals. Protocols are given for measuring the parameters that showed statistically significant changes in pesticide treatments. The most efficient experimental design would involve observing as many animals as possible, with one observation per animal. Parameters most likely to be useful in future studies are velocity, turning angle, upward and downward angles during hops, variance in vertical position, and hopping frequency. The parameters varied in usefulness, defined as the ratio of within‐treatment to among‐treatment variance (Fratios). Velocity was the most useful (i.e., had the highestFratio); hopping frequency was the least useful.Daphniaexhibited three kinds of swimming behavior, as defined by eight parameters: (a) “spinning” (extreme and continuous escape response) to acutely toxic levels of Carbaryl (40 ppb); (b) “irritation” (an increase in escape‐like behavior) to low levels of Carbaryl (1 ppb); and (c) “null” behavioral responses toChaoborus‐conditioned water, in which there was no detectable change in any measurement of swimming behavior. In predation trials, bluegill sunfish preferentially selected individuals showing “spinning” behavior. Results from this and other studies ofDaphniaswimming behavior suggest a conservative‐swimming hypothesis for understanding constrain