ABSTRACT: Mysids (Peracarida: Mysidacea) are standard test organisms used routinely in acute tests to evaluate the comparative toxicity of chemicals to aquatic organisms. The criticism of such lethal measurements for ecosystem monitoring has resulted in aclear need for sub-lethal tests that enable a more comprehensive understanding of the potential impacts of toxicants on natural populations. Neomysis integer, the dominant mysid of the upper reaches of European estuaries, has relatively low 96 hLC₅₀ values for pesticides and trace metals, and is under investigation as a suitable species for monitoring the pollution status of European estuaries. In this paper, the effects of environmentally realistic concentrations of theorganophosphate pesticide chlorpyrifos on some basic physiological processes of N. integer are evaluated as potential end points for indicators of environmental pollution. Oxygen consumption, egestion rate (used as a measure of feeding rate) andscope for growth (an integrated measure of the overall physiological status of an organism) were significantly affected by chlorpyrifos exposure. In contrast, absorption efficiency was unaffected by pesticide exposure. Chlorpyrifos effects were mostpronounced following short (i.e. 48 h) compared with longer exposure periods (e.g. 168 h). Increased oxygen consumption was the most sensitive toxicity test end point for detecting chlorpyrifos exposure, however, this response may not apply to alltoxicants. Reduced scope for growth and egestion rate are considered more representative of the effects of chlorpyrifos on natural mysid populations, but were only affected significantly at near-lethal exposure concentrations. All 3 responses wereaffected at concentrations lower than those currently acceptable by European Community legislation for potable waters.