Reports from anglers and divers indicated that in late August and September 1978 a substantial mortality of the bottom fauna and fish had occurred in the bays along the south coast of Cornwall, particularly in the St Austell area. It was therefore decided to carry out an investigation by diving and at the same time collect water samples from affected areas for further analysis. The following notes record the observations made during the period 19–28 September at various localities but mainly in St Austell Bay (see Fig. 1).