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, Cu 2 + and pentachlorophenol (PCP). Sublethal concentrations of Cu 2 + and PCP in ISO standard medium resulted in a linear decrease of the phototactic behavior of clone C 1 34 after 3.30h. The detection limit for changes in phototactic behavior of clone C 1 34 was 0.045mgl - 1 for Cu 2 + and 0.8mgl - 1 for PCP.