THE PRESENT study evaluated ethanol, methanol, ethyl acetate, hexane, chloroform and acetone extracts of five green and red seaweed species from Abu-Qir bay, Alexandria, Egypt for their antimicrobial, antioxidant activities and cytotoxicity against four cell lines. Chloroform extracts of Ulva lactuca and Ulva fasciata exhibited the highest inhibition zones against the tested pathogenic bacteria (Klebsiella pneumoniae and Proteus mirabilis) and fungi (Aspergillus flavus, Aspergillus fumingatus and Aspergillus niger) as measured by disc diffusion method. The extracts of U. lactuca and U. fasciata showed the highest antioxidant activity (IC50 6.32±0.29mg/ml and 6.61±0.27mg/ml, respectively), using DPPH (2, 2- diphenyl-1- picrylhydrazyl) scavenging method and total antioxidant capacity assay (2.13 and 1.51mg ascorbic acid equivalent/ gram dry weight, respectively). Cytotoxicity test using MTT assay showed that U. lactuca extract had strong activity against MCF-7 and Hela cell lines (IC50 10.83±1.0, 12.43±1.3μg/ml, respectively), while U. fasciata had strong activity against PC3 and HePG2 cell lines (IC50 12.99±1.2, 16.75±1.5μg/ml, respectively). As analyzed by Ultraviolet spectra, Fourier Transform Infrared spectra and Gas Chromatography Mass Spectroscopy after purification by column chromatography, the most active antimicrobial fractions in U. lactuca and U. fasciata extracts include an aromatic compound that have different active group (-C=O, phenyl ring and –OH); its molecular weight of Di-isooctyl Phthalate equals 390.56g/mol and Butylated Hydroxytoluene equals 220.356g/mol, respectively. The studied seaweeds have distinct active metabolites being a promising source of antimicrobial and antiproliferative compounds that can be used effectively in pharmaceutical drug industry.