Pebble counts were performed to assess the bed surface grain size distribution at different habitats within a reach of the Big River near Washington State Park, Missouri. These data were collected in support of research assessing the hydraulic conditions of freshwater mussel habitat in the Big River. One component of this investigation was to examine the theoretical stability and instability of sediment particle sizes that are present within mussel habitats as well as within nearby reference habitats. These pebble counts furnished grain size data used for reference sediment stability predictions. Pebble counts were performed at seven different habitats in a study reach on the Big River following the methods of Wolman (1954). Grain diameters (n ≥ 100) were measured on the intermediate axis using a ruler or calipers. Grains smaller than 2 mm were classified as 2 mm in size. Pebble count locations were classified to different habitat types based on the stream habitat classification developed by Panfil and Jacobson (2001).