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Abstract, Elevated phosphate concentrations at Safaga and Quseir, Red Sea, Egyptian coast, were related to some ecological and nutritional aspects of the macro-alga Cystoseim mvrica during 1985. The results obtained were compared with those recorded for a non-polluted control site at Ghardaqa.Chemical analysis of surface seawater samples revealed that the concentrations of dissolved phosphate at Safaga and Quseir were 3 and 20 times as much as that at Ghardaqa, respectively, The standing crop of C. mvrica showed two peaks in spring and autumn. Maximum biomass yield (491 g -).-. .m -) and number of m&vlduals ( 127 m -2) of C. myrica were recorded in the spring at the Ghardaqa site. Quseir C. mvrica samples contained significantly higher protein-N, totaI-N and total@ values but lower carbohydrate values than the Safaga and Ghardaqa samples (LSD at 1% level). Tissue N:P molar ratios for C. myrica, lxmrencia papillosa and Uh,a lactuca collected froln Ghardaqa ranged from 68-98, compared to 30-59 for the same species collected at the phosphorus polluted sites at Safaga and Quseir. The carbon levels and the C:N ratios of C. myrica exhibited no significant variations in the three studied sites. However, the C:P ratios of Ghardaqa plants were significantly higher than those of Safaga and Quseir.