The cyanobacterium Lyngbya majuscula was found to form mats on the submerged pneumatophores and leaves of Mangrove plants in the red sea. Aqueous and methanolic extracts of L. majuscula mats were bioassayed for their bioactivity against mice, Artemia salina, fungi and bacteria. The results of this study revealed that both aqueous and methanolic extracts were not toxic to mice at a concentration of 0.5g/ml. The aqueous extract did not have any effect either on Artemia or bacteria. The methanolic extract of this cyanobacterium exhibited a toxicity against A. salina with LC50 value of 0.3 g/ml. Also, the methanolic extract inhibited the growth of Bacillus subtilis, E. coli and Pseudomonas aeruginosa with minimum inhibition concentration (MIC) of 0.3, 0.2, 0.45 g/ml, respectively. Both aqueous and methanolic extracts with MIC of 0.13 g/ml had antifungal activity against all tested fungi, except Botryodiplodia which has been affected only by methanolic extract. In conclusion, the data point out that the methanolic extract of L. majuscula has antibacterial and antifungal activities, while aqueous extract has antifungal activity only. This finding indicates that the marine cyanobacterium L. majuscula could contain more than one bioactive substance.