In aiming to reduce the duration of measurement, a four‐channel dynamic light‐scattering system was developed. The four independent lights scattered at a fixed angle from a sample were detected in homodyne mode by a four‐channel photon counter. The accumulation and analysis of the data were carried out with a microcomputer. By this four‐channel system, swimming speed of bacteria was measured within 15 s with a statistical error of a few percent.