In vitro screening using methanolic extracts of different marine algae and one sea grass showed antibacterial activity against eight virulent strains of fish and human pathogens. 1:1 (V: V) ratios of each methanolic extract were mutually added to give 21 interactions. The interaction between Udotea sp and Codium fragile extracts exhibited higher activities against Edwardsiella tarda (18±0.9mm), Vibrio ordalli (16±0.8mm), Klebsiella pneumonia (20±1mm) and Pseudomonas aeruginosa (16±0.8mm). The bioactivity of this interaction was found to be more effective than that obtained from some common antibiotics including: Lincolin, Streptomycin and Gentamycin. The chemical constituents of these two algal extracts and their interaction were characterized using thin layer chromatography and gas chromatographic – mass spectral analysis. The bioactivity of the separated compounds was estimated. Palmitic acid was the most active fatty acid.