The effect of lysogenization with five temperate phages from various sources on serotype and lytic phage sensitivity was investigated in six cultures of Vibrio cholerae of both classical and El Tor biotypes. No changes in serotype or in classical phage sensitivity in the classical biotype were observed. Four of the temperate phages were homoimmune and induced resistance to one of the El Tor typing phages, E3, thereby causing a type change in El Tor strains. The sensitivity to the other phages was not changed. In 14 natural isolates too, E3 (group III) phage resistance correlated with the presence of temperate phage. Postadsorption exclusion was found to be the mechanism of resistance involved. The fifth phage, VcA-1, had a unique immunity profile. It could infect the El Tor biotype of V. cholerae but caused no change in the host properties investigated.