An optical lever was designed for studying physical displacements associated with electrophysiological activation of lobster nerve bundles. Stimulation current pulses generated a compound action potential volley, and upward physical displacements of <1 nm were recorded. The swelling displacement propagated in the same direction as the action potential volley, occurred simultaneously with the action potentials, and required 10 ms to relax after the electrical potential was restored. For comparison with previous reports, we also recorded the displacement of Nitella internodes associated with electrical stimulation. We found that a rapid swelling displacement (approximately 10 nm) was followed by a larger, slow-shrinking displacement (approximately 100 nm).