Three components of mean velocity have been measured in a square-sectioned 90 degree bend of 2.0 radius ratio with upstream and downstream straight ducts attached using rotable hot wires. Measurements were made for a Reynolds number of 40, 000 and in various cross-section planes. The contours of the longitudinal velocity and the velocity vector diagrams of the secondary flow were obtained at successive streamwise stations. The main conclusions derived from the experimental results are as follows : (1) The flow is biased towards the inner-radius wall of the bend over the range from the entry of the bend to the streamwise coordinate θ≒80°, θ being an angle measured from the entry of the bend in the streamwise direction ; (2) On the contrary, the flow is biased towards the outer-radius wall at θ<gsim>80° and the bias is the most remarkable just behind the exit of the bend ; (3) The intensity of the secondary flow induced in the cross-section is maximum near the exit of the bend.