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The grazing behaviour of Diadema setosum Leske and Heliocidaris erythrogramma Lamarck, on species of marine macro-algae has been studied in the laboratory. D. setosum showed strong preference to Galaxaura oblongata (Soland.) Lamx., while H. erythrogramma exhibited strong preference to Eucheuma striatum Forsk.) Schmitz. A generalized ranking of the algae used in the experiment from most to least preferred for both urchins included: Galaxaura oblongata, Padina gymnospora (Kutz.) Vick., Halymenia venusta (Forsk.) Borgesen, Dictyota sp., Eucheuma striatum, Gracilaria crassa (Forsk.) Harvey and Cystoseira trinodis (Forsk.) G. Agardh for Diadema setosum and Eucheuma striatum, Galaxaura oblongata, Dictyota sp., Padina gymnospora, Hormophysa triquetra (C.Ag) Kutz., Gracilaria crassa and Hypnea cornuta (Lamouroux) J. Agardh, for Heliocidaris erythrogramma. There was sustained avoidance for Sargassum aquifolium (Turn.) J. Agardh, Turbinaria sp., Boodlea composita (Harv.) Brand and Chaetomorpha crassa (C. Agardh) by both urchins. The agronomic implication of the feeding habits of the herbivores is indicated.