SUMMARY1. The objective of this paper was to reveal the toxicity of Fe3+and Fe24at pH 4.5 and 7 on larvae of the mayflyLeptophlebia marginata, by examining survival, motility, gill ventilation, moulting and feeding in experiments.2. Fe2+was the dominant metal species at pH 4.5, and Fe3+at pH 7. Precipitation of Fe occurred only at pH 4.5, where Fe‐precipitarions were observed on the thorax and the gills of the larvae.3. Both feeding activity and motility of the animals decreased at pH 4.5 and 10, 20 or 50mg1‐−1Fetot. After a short period of normal feeding, the animals stopped feeding for approximately 2 weeks and did not start to feed again until the end of the experiment. They were constipated. Survival was>95% in all treatments, except at pH 4.5 and 50 mg Fetot. In this group, about 20% of the animals died after having been constipated for 2