Streaky structures have been studied in a fully developed turbulent square-duct flow by using combined dye flow visualization-LDV velocity measurement techniques. Transverse distributions of low-speed streaks and mean streak spacing are shown in the near-wall region, i.e., yuτ/v < 25. At z/H < 0.16, the transverse distribution of the low-speed streaks is homogeneous and their spacing is about 90v/uτ, which suggests that the streaky structures near the wall bisector may be similar to those in turbulent boundary layers. In addition, the transverse distance between the visually identified low-speed streak and the velocity-measurement point was measured for each set of visual data. Conditioning the velocity fluctuations on this transverse distance, conditional probability density distributions of the axial, wall-normal and spanwise velocity fluctuations are shown. These distributions suggest the existence of quasi-streamwise vortical motions associated with the visualized streaky structures, their wall-normal dimension and the intermittency of their appearance.