A laboratory study on the sublethal effects of malathion on the net-spinning behavior of the caddisfly larvae Hydropsycheslossonae was conducted in order to assess the potential of net anomalies as an indicator of chronic exposure to organophosphorus insecticides. Two anomalies were identified after chronic exposure to 0.01, 0.05, 0.1, 0.5 and 1.0 μg l⁻¹ malathion. The first was a distortion of the midline meshes where the normal diamond shape structure was disrupted and the meshes were separated by extra strands (called ‘midline’ anomaly). The second aberration observed was a significant decrease in net symmetry. Both anomalies were highly correlated to the toxic action of malathion, i.e. inhibition of the acetylcholinesterase enzyme (AChE). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses of capture nets did not show any modification of silk polypeptides after exposure to malathion, indicating that net distortions were not related to silk composition. Both anomalies seem to represent the symptoms of the specific toxic action of malathion; nevertheless, they can serve as an index of the physiological condition of the larvae, especially the midline anomaly. The symmetry of the nets decreased significantly after exposure to 0.5 and 1.0 μg l⁻¹. However, the toxicity curves (EC₅₀) showed that the sensitivity threshold for the midline anomaly ranged from 0.11 to 0.28 μg l⁻¹, which reflect more realistic exposure to concentrations expected to occur in the field. Hence, the use of capture net anomalies of hydropsychid larvae could represent a valuable indicator of sublethal toxicity induced by malathion and other organophosphorus insecticides in running waters.