We tested the hypothesis that the use of a positively phototactic Daphnia magna clone can reduce the background noise and thereby increases the sensitivity of a continuous biomonitor that uses changes in phototactic behavior as a variable for the detection of stress caused by pollutants. We selected D. magna genotypes with a stable and predictable phototactic behavior in presence and absence of fish kairomones. Two clones were positively phototactic in absence of fish kairomones (clone C(1)34 and C(1)242). and two clones (C(1)242 and C(1)31) showed no significant change in behavior in response to Fish kairomones. Using a simple experimental set-up, we quantified the changes in phototactic behavior of adult animals of the positive phototactic clone C(1)34 in response to two standard pollutants, Cu2+ and pentachlorophenol (PCP). Sublethal concentrations of Cu2+ and PCP in ISO standard medium resulted in a linear decrease of the phototactic behavior of clone C(1)34 after 3.30 h. The detection iii-nit for changes in phototactic behavior of clone C(1)34 was 0.045 mg l(-1) for Cu2+ and 0.8 mg l(-1) for PCP. (C) 1998 Elsevier Science Ltd. All rights reserved.