Intracellular development of choleraphage Φ149 following infection of Vibrio cholerae and Vibrio eltor cells under different conditions was examined by thin-section electron microscopy. Degradation of the host DNA following infection and formation of mature phage particles inside the infected cells were demonstrated. The concatemeric DNA intermediate formed during intracellular replication of Φ149 DNA in permissive hosts was resolved. In confirmation of biochemical evidence, no concatemeric DNA intermediate was observed for infection in high-phosphate mediu