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The biological effects of three Seaweed extracts Seagrass Thalassia hemprichii, Sea lettuce Ulva lactuca, and Gray Mangrove Avicennia marina against 4th instar mosquito larvae of Anopheles d’thali were evaluated. Values of IC50 (concentration which to inhibit the emergence of 50% of mosquito adults survived from larval treatments) indicated that A. marina (107.3 ppm) proved to be the most effective extract against An. d’thali, followed by T. hemprichii (197.3 ppm) while the seaweed extract U. lactuca (335.3 ppm) was the least effective one. Variations in susceptibility levels of An. d’thali mosquito larvae may probably due to differences in toxicity levels among the active components of seaweed extracts. On the other hand, treatments with IC90 values of the test extracts A. marine (284.7 ppm), T. hemprichii (293.8 ppm) and U. lactuca (588.3 ppm) did not have any adverse effects towards some aquatic nontarget organisms (Odonata naiads, aquatic hemipterous, aquatic coleopterans) prevailing in mosquito breeding sites.