Erosion of banks and dikes subjected to water motion can be prevented, for instance by application of cellular concrete revetment blocks. This type of revetment combined the merits of a closed block revetment and a slope revetment consisting of loose materials. Advantages are high permeability, high stability against wave attack and low block weight. On top of this, cellular blocks are appealing because they allow the growth of vegetation. It is of vital importance that the granular material in the holes does not erode completely. Therefore physical model tests were carried out. The paper describes these tests and gives results.