The rate of deterioration of gravel base course materials may influence the durability of flexible pavements. In particular, selected gradation curves usually satisfy filter criteria thus avoiding migration of fines from one layer (subgrade, base, and subbase) into adjacent one. This paper shows that even if filter criteria are met between successive layers, some 0–20 mm materals do not satisfy the fines mobility criteria that presently are not considered in gradation criteria. This means that fines (<80 μm) can move within the pores of the layer. Three gradations of two 0–20 mm crushed stones were tested in the laboratory. Experiemrnts were designed to avoid degradation (breakage of particles), in order to study segregation only. One of the tested materials (No. 2) satisfies the gradation criteria but does not satisfy the fines mobility criteria. The experimental results confirmed that material No. 2 is subject to marked segregation during dry placement and compaction. When palced moist, it has a negligible initial segregation, but cyclic loading produces segregation. This internal instability was predictable from fines mobility (suffossion) criteria. It produces layers with different fines contents, which would reduce vertical drainage, increase pore pressures produced by traffic loading, and increase both capillary retention and frost damage. Key words: road, gravel base course, segregation, suffossion, seepage, gradation.