It was shown a tetranucleotide, CGCG, to inhibit infectious activities of some algal viruses by 102-104 times. This process depends on the duration of CGCG-virus contact. The results obtained seem to be interesting from the practical point of view and may be as a foundation of hypothesis of the role of DNA decay products appearing in the aquatic environment as a result of viral lysis of unicellular hosts in the smooth stop of active viral infection. This assumption does not contradict the conceptual model of virioplankton control of host community diversity by Wommack and Colwell, but complements it.