Background: aqueous extracts of six species of marine seaweed were studied as antiviral activity on different viruses. Materials and methods: these collected from two sites Hurghada at the Red Sea and Al-Agami area in Alexandria Mediterranean Sea Egypt and belonging to the classes Chlorophyta, Phaeophyta and Rhodophyta were assayed for the cytotoxicity and antiviral activity by MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenlytetrezolium bromide] and by neutralization methods. Results: these extracts have antiviral activity to herpes simplex virus types-1 (HSV-1) and type-2 (HSV-2), hepatitis A virus (HAV-H10), and Coxsackie B4 virus in Vero cells with very low cytotoxicity to the host cells.