This article describes electrooptical (EO) characterization of biospecific binding between the bacterium Escherichia coli XL-1 and the phage M13K07. The electrooptical analyzer (ELUS EO), which has been developed at the State Research Center for Applied Microbiology, Obolensk, Russia, was used as the basic instrument for EO measurements. The operating principle of the analyzer is based on the polarizability of microorganisms, which depends strongly on their composition, morphology, and phenotype. The principle of analysis of the interaction of E. coli with the phage M13K07 is based on registration of changes of optical parameters of bacterial suspensions. The phage-cell interaction includes the following stages: phage adsorption on the cell surface, entry of viral DNA into the bacterial cell, amplification of phage within infected host, and phage ejection from the cell. In this work, we used M13K07, a filamentous phage of the family Inoviridae. Preliminary study had shown that combination of the EO approach with a phage as a recognition element has an excellent potential for mediator-less detection of phage-bacteria complex formation. The interaction of E. coli with phage M13K07 induces a strong and specific EO signal as a result of substantial changes of the EO properties of the E. coli XL-1 suspension infected by the phage M13K07. The signal was specific in the presence of foreign microflora (E. coli K-12 and Azospirillum brasilense Sp7). Integration of the EO approach with a phage has the following advantages: (1) bacteria from biological samples need not be purified, (2) the infection of phage to bacteria is specific, (3) exogenous substrates and mediators are not required for detection, and (4) it is suitable for any phage-bacterium system when bacteria-specific phages are available.