Different extracts from marine algae (Cystoseira tamaricifolia) were tested for their antimicrobial activities. These extracts were examined for their activities on growth of moulds and yeasts and mycotoxines formation by moulds. The Minimal Inhibitory Concentration (MIC) was determined in every case in terms of dilutions. A total inhibition (100%) was obtained with the ethanolic crude extract on all the fungal species at the concentration of 10%. The aqueous extract showed an activity only on some strains used while no activity was observed on yeasts and moulds for both methanolic and aqueous extract. The other extracts (chloroform, hexane and methanol) had no activity on the microbial growth. Mycotoxins formation in Aspergillus flavus was inhibited by the ethanolic extracts at the concentration of 5%.