In the seawater of the Atlantic Ocean and surface waters of different points of the World Ocean, including arctic basins and equatorial regions, microvibrion-predators of bacterial cells have been discovered. The number of microvibrions in the surface waters of the English Channel reaches a thousand cells per ml, but it varies, depending on the season. In this article a method of the separation and culture of microvibrions is described. The ultrastructure of two strains of microvibrions was investigated and a similarity found between their structure and Gram-negative bacteria. The distinctive features of microvibrions are their sea derivation and the ability to reproduce only in the presence of bacteria, without penetrating bacterial cells. To a certain extent, the antibacterial activity of seawater is connected with the presence in it of microvibrions.