We report in this study the isolation of marine bacteria with antiviral properties that have been tentatively classified as Moraxella. These bacteria retained their virucidal capacity after prolonged subcultivation in the laboratory. The virus-inactivating agent could not be separated from the viable marine bacteria, indicating that the active agent(s) either remains associated to the microorganisms or has a very short lifetime, or both. The antiviral capacity of the isolated microorganisms was highly specific for poliovirus. No virucidal effect was observed against other strains of enteroviruses, such as Coxsackie and ECHO virus, rotavirus SA11, or bacteriophages proposed as indicators of the virological quality of water, such as coliphage f2 and bacteriophage B40-8, which infects Bacteroides fragilis.