Increasing sediment inputs are recognised as an important factor leading to coral reef degradation. However, the role of sediments in ecological processes is poorly understood. This study used paired-choice trials to quantify the effects of sediment grain size and chemical composition on feeding by the abundant detritivorous reef fish, 'Ctenochaetus striatus'. The size of sediments from algal turfs were also compared to those ingested by reef-dwelling 'C. striatus'. Algal turfs containing coarser sediments were preferred by 'C. striatus', while sediment composition (reefal carbonates vs. riverine silicates) had little effect. On the reef, 'C. striatus' ingested finer sediments than those present in algal turfs. 'C. striatus' appears to prefer algal turfs with coarser sediments as this facilitates ingestion of fine detrital particles, while finer sediments prevent selective feeding on detritus. These findings suggest that fine sediments from terrestrial runoff or dredging may be detrimental to feeding by detritivorous species.