Abstract The establishment of a pseudolysogenic state accompanied by a phenotypic conversion in Azotobacter vinelandii strain O by phages A14, A21, A31, and A41 has been identified. Host cells can be recovered from the pseudolysogens by cultivation in phage-specific antiserum. Pseudolysogens continually give rise at a low rate to phage as a result of the occasional initiation of a lytic burst. As a result of the establishment of the pseudolysogenic state the host cells lose their polysaccharide coat, become flagellated and motile and acquire a yellow pigmented appearance. The four phages, although they differ serologically and molecularly, give rise to converted states that are indistinguishable except by the identification of the phage that is produced by it. On repeated subculturing each of the pseudolysogens will give rise to a stable or permanently converted cell that has the phenotype of the pseudolysogen, but from which it is no longer possible to obtain either host cells or phage.