Gene 18 of bacteriophage T4 encodes the contractile protein of the tail sheath. Previous work has shown that the full-length recombinant gene product (gp) 18 of 658 amino acid residues assembles in Escherichia coli cells into a long polysheath structure. However, the gp18 mutants truncated at the N-termini form insoluble aggregates similar to inclusion bodies. In this study, six plasmid vectors expressing the recombinant gp18 proteins truncated at the C-termini have been constructed. The C Delta 58, C Delta 129, C Delta 152, C[g1]72, C Delta 48, and C Delta 287 proteins contain 600, 529, 506, 486, 410, and 371 residues of the full-length gp18 molecule, respectively. All the recombinant proteins were soluble and, except for the C Delta 287 mutant, were assembled into polysheath-related structures. Electron microscopy of negatively stained purified proteins was performed and the resulting images were analyzed by computing their Fourier transforms. The C Delta 58 and C Delta 129 mutants, in addition to forming common contracted-type polysheath structures, assembled into thinner filaments that we called "noncontracted polysheaths" (NCP). The C Delta 152, C Delta 172, and C Delta 248 proteins assembled into the NCP type only. Image processing showed that the NCP filaments significantly differ from both extended sheaths of T4 particle and polysheaths. The structure of the NCP filaments might correspond to the transitional helices postulated by Moody (J. Mol. Biol., 1973, 80, 613-636) that appeared during the process of tail contraction. Our results suggest that a short region at the C-terminus of the C Delta 129 protein determines the contractile properties of the gp18 molecule. The shortest, the C Delta 287 protein, does not assemble into regular structures, thus indicating that a sequence's stretch at the C-end of the C Delta 248 mutant might be responsible for polymerization of gp18.