Abstract Electron microscopic studies indicate that λ head morphogenesis involves an initial core-containing prohead I which converts to an empty prohead II which is subsequently filled with DNA to form the mature λ capsid. This scheme is based on observations of thin sections through samples of λcI857 lysogens fixed at various times after induction. The proportions of the different head-related structures change as a function of time after induction so that prohead I is initially (18 min after induction) the predominant form; later (23 min after induction) prohead II is the predominant form. By 42 min after induction, full heads account for about 45% of the head-related structures, prohead II for approximately 45%, and prohead I for 10%. Studies of various λ and Escherichia coli mutant lysogens show that (1) λA − , λD − , and λB − mutations cause the accumulation of prohead II; (2) λC − and λC − B − mutations lead to the accumulation of prohead I structures, some of which have off-centered cores; (3) E. coli groE − mutations block capsid assembly at the prohead I stage; (4) λNu3 − and λNu3 − C − mutations lead to the production of some headlike objects as seen in negatively stained lysates; however, in thin sections the normal head-related structures are not seen, but malformed, indistinct structures are visible. The head-related structures that accumulate in E. coli groE − , and λC − inductions contain pNu3 (Hendrix and Casjens, 1975); these structures correspond to the core-containing prohead I. The head-related structures found in λA − and λB − inductions lack pNu3; these structures correspond to prohead II. Our data suggest that pNu3 may be a component of the prohead I core which is required for normal λ head assembly, and that pC may act as a linker between the core and the outer capsid shell.