The aim of this work was to determine the effect of water activity on structure and mechanical properties of rye crisp bread and fibre crisp bread containing wholegrain rye flour, wheat bran, oat meal and sesame seeds. Breads were stored at water activities in the range of 0-0.75. Their mechanical properties (deformability modulus, fracture stress and strain, fracture work) were measured by three-point bending tests. The microstructure of the crisp breads was studied at two water activities by scanning microscopy. Water activity significantly influenced mechanical properties of crisp bread. Increasing water activity caused an increase of the fracture stress at water activities ranging from 0.030 to 0.255 for rye bread and at a_w 0.039-0.319 for fibre bread. An increase of water activity above these values caused softening and a sharp decrease of the deformability modulus and fracture stress. Microscopic photographs showed that water activity had an influence on the structure of crisp bread. The rye bread had lower resistance to deformation than the fibre bread, probably due to differences in composition, and the added grain ingredients gave a more heterogeneous structure.