This study documents the effects of short-term (24 h) sublethal copper exposures on undirected swimming activity and photobehavior of Balanus improvisus stage II nauplii. All Cu treatments were static, with temperature and salinity conditions at 20 C and 15 or 30 percent. The 24 h LC 50 estimate for Cu is 88 ppb at 15 percent and > 220 ppb at 30 percent. Sub-lethal Cu concentrations cause reductions in swimming speed, which decrease progressively with increasing Cu dose. At 50 ppb Cu, this was significant primarily at light intensities below the phototactic threshold. At high Cu concentrations, significant reductions in mean linear velocity occurred at most light intensities tested. At 30 percent, 50 and 100 ppb Cu also reduce the positive phototactic response and 150 ppb Cu causes reversal of phototaxis at optimal intensities. Photokinesis is reduced at 100 ppb Cu and disappears at 150 ppb Cu. At 15 percent, the behavioral effects of 50 ppb Cu resemble those occurring with 150 ppb Cu at 30 percent. Swimming speed and photobehavior show promise as sensitive behavioral indicators of copper toxicity. Additional research is required to determine if these responses apply to a broad range of pollutants and to other planktonic organisms. There is also a need to further evaluate the significance of these behavioral effects ecologically.