The bacteriophage P22 has a short tail with a hexagonal base plate but no contractile sheath. It is not adsorbed on a mutant strain ( St 22 ) of its host Salmonella typhimurium ( St). A mutant P22 h, forming faint plaques on St 22 and clear plaques on St, is indistinguishable morphologically from P22. Preparations of P22 also contain a small proportion of a long-tailed phage, P221, which forms plaques on St 22 but not on St, by which it is not adsorbed. Different strains of P22, containing markers like c +, c 1 and c 2 that affect lysogenization, produce P221 strains with corresponding markers. Neither P22 nor P22 h cross reacts serologically with P221. However, mixed infection of St 22 by P22 h and P221 produces masked genomes: particles carrying P22 h genomes in P221 capsids and P221 genomes in P22 h capsids. Moreover, P22 h markers can be transferred to P221 genomes and vice versa. The existence of these stable hybrids may indicate that P221 represents a morphologically and serologically distinct mutant of P22. Alternatively, there may be present in St a defective prophage whose defects can be rectified by recombination with P22 to yield P221.