Vickers microhardness indentations of 10 mu m (001) oriented epilayers of AlxGa1-xAs on GaAs substrates have been utilized to evaluate the hardness H-v, the internal stress, and the fracture toughness K-lc of the layers as a function of their composition parameter x. The hardness H-v varies linearly according to : (6.9 - 2.2x) GPa and K-lc increases linearly with x according to : K-lc = (0.44 + 1.30x) MPa m(1/2). The influence of the substrate on these measurements was found to be negligible for the layer thickness (10 mu m) and the indentation load (0.25 N) used, disregarding internal stresses. Internal film stresses were evaluated by the bimorph buckling method, and were found to depend on the composition parameter according to sigma = 0.13x GPa. These stresses did not notably affect the H-v measurements, but for K-lc corrections as large as similar to 25% had to be made. The radial cracks observed were of the shallow Palmqvist type. In contradiction to previous reports on this type of cracking, it was found to initiate during unloading, not during loading, and a physical explanation for this deviation is given. No deep radial/median cracks were observed. It was found important to use expressions based on the correct crack geometry in the K-lc evaluation. Also, a simple theory for the influence of internal stresses on the K-lc results has been developed.