Extinction contours observed in silicon whiskers in the electron microscope have been studied. The details of the extinction contours depend, in general, on the orientation of the whisker and the geometry of the reflection. Several interesting diffraction effects have been observed. The intrinsic angular width of the electron beam can be determined from the extinction contours. In some cases, the extinction contours outline the cross section of the whisker, revealing directly the morphology of the whisker.