With the stable environment afforded by a dynamically pumped wet specimen chamber, the thickness of a fully hydrated protein microcrystal may be obtained in an electron microscope from an incident‐beam attenuation measurement after disappearance of the diffraction pattern. Such measurements on orthorhombic catalase crystals have been successfully cross‐correlated with thickness measurements on the same crystals made with an interference light microscope over a range from a few hundred to a few thousand Ångst