Abstract The surface of canavanine-induced T4B and T4D giant phages was investigated with the electron microscope using freeze-etching and shadow casting methods. On the relatively smooth surface of the elongated phage heads, a left-handed helical arrangement of subunits of about 4.0 to 4.5 nm diameter was found with interhelical distances ∼6.7 nm and an angle of 15°, measured normal to the long axis of the particle. These helices were the most obvious; they were intercepted at about 60° by two other lattice lines, one of 75° relative to the equator, the other one at an angle of 45°. Both of these lattice lines were about equally spaced but of lesser definition than the first helix. This “smooth” surface pattern changed drastically after exposure of the phage to temperatures between 42 and 48° for 10 to 60 min; the interhelical distance then measured 11.1 ± 0.4 nm, and “capsomeres” of 7.0 to 7.4 nm diameter became visible, producing a rather coarse hexameric lattice. No significant change in the angles between the lattice lines was observed. During storage of suspensions of the “smooth” phage at 2 to 4°, most virus surfaces had assumed the “coarse,” 11.1 nm arrangement. By employing acrylamide electrophoresis, we found that the heat treatment releases a protein with molecular weight of about 11,000. Lollipops of T2L, a phage lacking a capsid protein of similar size, revealed an ill-defined but “coarse” lattice which did not change after heat treatment.