The effect on phage morphogenesis of sus mutations in the cistrons coding for nonstructural proteins has been studied. Mutants in three cistrons analyzed that are involved in phage DNA synthesis, as well as in cistron 16 which codes for a late nonstructural protein, produce prolate capsids which are more rounded at the corners than complete phage heads and have an internal core; they contain the head proteins, the upper collar protein and protein p7, not present in mature phage particles. Mutants in cistron 7 do not produce capsids nor other phage-related structures; this result and the presence of p7 in phage capsids suggest an essential role in capsid assembly for this protein. The protein product of cistron 13 is probably needed for a stable DNA encapsulation since mutants in this cistron produce mainly DNA-free complete phage particles and only about 10% of uninfective DNA-containing complete phage. Cistron 15 codes for a late, partially dispensable, nonstructural protein which is present in the DNA-free capsids produced after infection with the delayed-lysis mutant sus14(1242), used as the wild-type control, or with mutants in cistrons 9, 11,12 and 13. Proteins p15 and p16 are probably involved in the encapsulation of viral DNA in a prohead.