Continual application of chemical insecticides causes a lot of environmental and health problems. Mosquitoes as vectors of diseases develop resistance toward them. Finding out safe alternatives is an urgent need. Algal extracts are a promising alternative approach as they are biodegradable and eco-friendly. The present study assessed the larvicidal efficacy of three ethanolic algal extracts (Chaetomorpha linum, Ulva intestinalis and Sargassum dentifolium) belonging to two different algal divisions against the 3^rd larval instar of Culex pipiens L. (Diptera: Culicidae) under laboratory conditions. Toxicological studies showed that three species of brown and green algal extracts exhibited good larvicidal activity. Percentages of mortality increased with increasing concentrations of all algal extracts. Based on the LC₅₀ values, C. linum (Chlorophyta) exhibited the highest larvicidal potency among the used extracts, followed by U. intestinalis (Chlorophyta) and S. dentifolium (Phaeophyta) with LC_{50, s} of 224.45, 231.06 and 241.79 ppm, respectively, upon 48 hours exposure. GC-Mass analysis of the tested extracts revealed that the most abundant constituents in the ethanolic extracts of C. linum, U. intestinalis and S. dentifolium were palmitic acid (17.42 %), linolenic acid (23.93%) and Di-n-octyl phthalate (19.03 %), respectively. The application of LC₅₀ values of tested ethanolic algal extracts induced many biochemical and morphological aberrations in the treated C. pipiens larvae, compared to the untreated larvae. The biochemical changes were tracked through the analysis of the insect's main metabolites (carbohydrates, lipids and proteins), in addition to measuring the changes in acetylcholine esterase, GST, α- and ß- esterases after algal extracts treatment. Generally, variations were recorded in the lipid and carbohydrate after treatment with C. linum. Moreover, C. linum inhibited the activity of α and ß esterases enzymes to a great extent, compared to the untreated. Many histological abnormalities were noticed in the treated larvae. Overall, these results demonstrated that ethanolic algal extracts of C. linum, U. intestinalis and S. dentifolium might be used to control C. pipiens mosquitoes without harming humans or the environment. Hence, they could be incorporated into integrated vector management programs.