We study the effects of polluents (dose-response relation) and other stress factors (e.g. crowding) on the phototactic behaviour of the waterfela Daphnia, aiming at the development of a continuous biomonitor. In experiments with extended exposition to heavy metals, we will also study acclimatization. Experimental crossing and selection will aim at selecting an ideal genotype (clone) for us in a biomonitor: this clone should show a positively phototactic behaviour, be insensitive to fish kairomones and show a reduced capacity to acclimatize to pollulants.