Several commonly used procedures for sampling coarse riverbed material result in samples that are biased toward the larger particles. Kellerhals and Bray (1971) recommend correcting the bias by scaling the sample frequency of each frequency interval by the inverse of the appropriate power of the mean grain size of the interval. If, however, grain size is assumed to be log normally distributed, it is possible to determine the distribution of the biased sample. This provides explicit bias factors for the sample moments.