An analysis of digitized three-dimensional video records of free-swimming Daphnia pulex identified 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 (F ratios). Velocity was the most useful (i.e., had the highest F ratio); hopping frequency was the least useful. Daphnia exhibited three kinds of swimming behavior, as defined by eight parameters: (a) spinning (extreme and copntinuous 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 to Chaoborus-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 of Daphnia swimming behavior suggest a conservative-swimming hypothesis for understanding constraints on Daphnia swimming behavior.