The acanthurid fish species Ctenochaetus striatus and Acanthurusnigrofuscus are common detritivore browsers. Less is known about theirerosion potential. We offered coral limestone for grazing in 24haquariumexperiments and recorded the feeding behaviour by video. C.striatus produced 15 times more calcareous sediments (faeces and loosematerial) compared to A. nigrofuscus despite of a 5 times higher bitingrate of A. nigrofuscus. This discrepancy can be explained by differentfeeding techniques of the species. A. nigrofuscus performed carefulpicking bites only, whereas C. striatus showed a more differentiatedfeeding behaviour: soft sweeping of the surface alternating with chafingthe substrate with grasping bites at which the whole body was shakingand exerting trust. Hereby a special palate structure (bow of hard knobsin the upper jaw) is engaged which allows to rasp the reef substrate(Krone et al. 2006). The measured eroded masses per bite and thenumber of bites per individual were related to the population densitiesof the two species on three reef crests in the Northern Red Sea. Based onthese calculations, A. nigrofuscus attains maximum erosion rates of 2.6 gm-2 yr-1 and may therefore not be regarded as a relevant bioeroder, but C.striatus achieves erosion rates of up to 70.0 g m-2 yr-1 which is in thesame order as that of the co-occuring sea urchin Diadema setosum.