This study was carried out to evaluate the antimicrobial activity of hexane, chloroform, ethyl acetate and water extracts of six marine macroalgae belonging to different families namely green algae (Chlorophyceae), brown algae (Phaeophyceae) and the red algae (Rhodophyceae) collected from the intertidal area of the Sudanese Red Sea coast near Port Sudan. Successive extraction techniques using hexane, chloroform, ethyl acetate and water were used for extracting the active principles of the algae and the disc-diffusion method was performed to examine the activity of the crude extracts against four pathogenic bacteria and one fungus. Most of the tested algal extracts exhibited considerable bioactivity and inhibited the growth of most of pathogenic microorganisms under investigation. The chloroform extract of the red alga Gracillaria sp produced a maximum inhibition zone of 22 mm against Escherichia coli whereas the chloroform extract of the red alga Laurencia papillosa showed a low antimicrobial activity with a minimum inhibition zone of 5 mm against Bacillus subtilis.