Freeze fracture and deep etching were used to investigate the ultrastructural basis for the observation that anti-treponemal antibodies bind poorly to the surface of virulent Treponema pallidum. Fractures of T. pallidum outer membranes contained scarce, uniformly sized intramembranous particles (IMPs). IMPs on the convex faces often appeared to form linear arrays that wound in spirals about the organism. In contrast to the outer membrane, IMPs of the cytoplasmic membrane were randomly distributed, numerous, and heterogeneous in size. In Escherichia coli and T. pallidum cofractures, IMPs of the E. coli outer membranes were densely packed within the concave fracture faces, while the T. pallidum fractures were identical to the experiments lacking the E. coli internal controls. Outer membranes of two representative nonpathogenic treponemes, Treponema phagedenis biotype Reiter and Treponema denticola, contained numerous IMPs, which segregated preferentially with the concave halves. Examination of apposed replicas and deep-etched specimens indicated that at least some of the IMPs extend through the T. pallidum outer membrane and are exposed on the surface of the organism. The outer membrane of intact T. pallidum appears to contain a paucity of integral membrane proteins that can serve as targets for specific antibodies. These findings appear to represent an unusual parasitic strategy for evasion of host humoral defenses.