The sediment sampling efficiency of two-dimensional slots in open channel was studied. Eight fractions of sand and gravel were used ranging in size from 0.2 mm to 4.5 mm. A universal relation between the efficiency, the Froude number and the particle size was established for what is termed the bed load transport range. It was found that the sampling efficiency in this range decreases slightly with particle size and approaches one for large slots openings. The sampling efficiency of smaller particles, which do not move as bed load, was found to increase with particle size but remained smaller than one even for extremely large slot size to particle size ratios.