Ten cyanobacterial species (Nostoc calcicola , N. commune, N. entophytum, N. minutum, N. palndosum, N. passerianum, N. punctiforme, Anabaena ambigua, A. amomala, and  A. doliolum) were isolated from the mangrove region of Ras Mohammed (Sinai, Egypt) and have been tested for their allelopathic activity that of inhibitory and / or promoting effects against two Gram positive bacteria (Bacillus subtilis and Staphylococcus aureus) and two Gram negative bacteria (Escherichia coli and Pseudomonas aeruginosa). Data suggested two types of allelopathic effects: one type which always appeared in cyanobacterial medium as in the case with Nostoc minutum (medium that inhibits the growth of all tested bacterial species). The other type is induced only when Cyanobacteria are in contact with bacteria; this is the case when the growth of both Bacillus subtilis and Staphylococcus aureus were inhibited in co-culture with Nostoc commune. On the other hand, promotion effects of bacterial growth were observed when grown in cyanobacterial metabolites in most of studied cyanobacterial species.The biological assays for aqueous and methanolic extracts of the two Nostoc species revealed that both extracts for each species were not toxic at concentrations of 0.52 and 0.59 g L^-1 water extract for Nostoc commune and N. minutum, respectively and 0.31 and 0.425 g L^-1 for methanolic extract for Nostoc commune and N. minutum, respectively. No mortality was observed in tested mice within 72 hours