The boundary layers of rainbow trout, Oncorhynchus mykiss [mean±s.d., 0.231±0.016 m total-body-length (L); N=6], swimming at 1.6±0.09 L s(-1) (N=6) in an experimental flow channel (the Reynolds number, Re=4×10(5)) with medium turbulence (5.6%-intensity) were examined using the particle image velocimetry technique. The tangential-flow-velocity distributions in the pectoral (arc-length from the rostrum, lx=71±8 mm, N=3) and pelvic surface regions (lx=110±13 mm, N=4) were approximated by a laminar-boundary-layer model, the Falkner-and-Skan equation. The flow regime over the pectoral and pelvic surfaces was regarded as a laminar flow, which could create less skin-friction drag than would be the case with turbulent flow. Flow separation was postponed until vortex shedding occurred over the posterior surface (lx=163±22 mm, N=3). The ratio of the body-wave velocity to the swimming speed was in the order of 1.2. This was consistent with the condition of the boundary-layer laminarisation that had been confirmed earlier using a mechanical model. These findings suggest an energy-efficient swimming strategy for rainbow trout in a turbulent environment.