The concentrations of metal contaminants often fluctuate in estuarine waters; yet we have limited knowledge about the effects of intermittent exposures on estuarine organisms. Using 10-d lethality bioassays with the epibenthic amphipod Melita plumulosa, different combinations of intermittent (pulsed) dissolved Cu exposure were investigated, varying Cu concentration, pulse duration, and time between pulses. Negligible organism mortality was observed immediately after single 12- to 62-h duration pulsed exposures of 100 to 900 mg/L dissolved Cu. However, delayed mortality was observed in the subsequent 96-h nonexposure period, after which negligible additional mortality occurred during the remainder of the 240-h tests. For multiple pulsed exposures, increasing the time between pulses from 0 to 144 h did not result in significantly different mortality rates for 300 and 400 mg/L dissolved copper, indicating that the organisms did not recover between pulses. Organism mortality exhibited a strong relationship with the time-averaged concentration (TAC) resulting from the combination of exposure concentration and duration. The lethal concentration to 50 (LC50), 20 (LC20), and 10% (LC10) (95% confidence interval) of the test population for the combined TAC exposure–survival data were 86 (71–103), 44 (30– 64), and 30 (18–49) mg Cu/L, respectively, which were similar to the respective values of 100 (87–114), 55 (43–70), and 39 (28–54) mg Cu/L determined for continuous exposure. The results from the current study support the use of analytical techniques capable of determining the time-averaged concentration of metals, because they will more accurately predict the effects of toxiciants on organisms than single time-point measurements.