The benthic amphipod Pontoporeia affinis is closely associated with the bottom and it appears that the nature of the substrate may have important influence on its distribution. In this study samples were taken from an area (ca. 54 km2) at the mouth of Grand Traverse Bay in northern Lake Michigan. The sampling depths ranged from ca. 20 m to ca. 120 m. The samples were analyzed as to grain size distribution and organic content of the sediments and to the abundance and size distribution of the amphipods associated with these sediments. The numbers of animals ranged from 1,293 per m2 to 14,221 per m2. The correlation of amphipod abundance with particle-size distribution is not close. This relationship is discussed with regard for the results of substrate-preferenda experiments conducted in the laboratory with organic-free sediments. The presence of organic material overlying the sediments, or on the surfaces of the sediment particles, has a positive influence on distribution and abundance. The relationship of distribution and organic content is discussed with regard for the feeding mechanism and habits of the animal in the laboratory. Substrate preferenda experiments are discussed as they relate to the distribution of P. affinis and as they further relate to organic material associated with the sediments as the nutrient source of this population.