We have screened for in vitro inhibition of viral replication with extracts from the following marine microalgae: Porphyridium cruentum, Phaeodactylum tricornutum, Tetraselmis suecica, Chlorella autotrophica, Dunaliella tertiolecta, Dunaliella bardawil, Isochrysis galbana, Isochrysis galbana var Tiso, Ellipsoidon sp. and Tetraselmis tetrathele. We have used as viral models two enveloped viruses of significant economic importance, the viral hemorrhagic septicemia virus (VHSV) of salmonid fish and the African swine fever virus (ASFV). The aqueous extracts from P. cruentum, C. autotrophica and Ellipsoidon sp., produced a significant inhibition of the in vitro replication of both viruses in a dose-dependent manner. That this inhibition could be due to sulfated polysaccharides was suggested because the same pattern of viral inhibition was obtained by using exocellular extracts from microalgae enriched in these compounds and/or dextran sulfate of high molecular weight. However, the inhibition of viral replication did not correlate with the percentage of sulfatation of the exocellular polysaccharides. Extracts from marine microalgae may have prophylactic utility against fish and mammalian viral diseases.