The pathogenic E. coli strain, serovar 0124 studied in this investigation and its mutants rec A 56 and rec A 441 differ in their ultrastructural organization. In strains having defects in gene rec A, the appearance of intracytoplasmic membranous structures and a great number of extracellular membranous vesicles, as well as the formation of the filamentous forms of cells in the colonies, can be observed. These data indicate that the product of gene rec A plays an active role in the metabolism of bacterial membranes.