There was little difference in estimates of viable populations of sulphate-reducing bacteria in sediments of the quiescent and turbulent areas of the reef off the south-east of Lizard Island. From [35S]sulphate and in situ measurements, the active sulphate-reducing zone was 0-3 cm below the surface of the lagoon sediment and the average rate of sulphide production was 67 mg S2- m-2 day-1 or 245 g S2- m-1 year-1. In quiescent sediments the highest sulphide concentrations were in the 1-3-cm zone below the surface and there was a significant positive correlation between sulphate-reducing activity and sulphide content of the sediments sampled. Precipitated sulphate was also found in all sediments with an average of 0.34% by weight.