Summary: Epididymite, determined by means of its optical properties, X-ray powder pattern, and spectrographic analysis, was found in a desilicated pegmatite near Věžná, Western Moravia, Czechoslovakia. It was formed contemporaneously with minor amounts of milarite in a late hydrothermal stage by the action of sodium-rich solutions on beryl. This is the first occurrence of epididymite and milarite as alteration products of beryl and the first findings of epididymite in any other paragenesis than that of alkaline pegmatites. The optical properties of epididymite are re-examined and older data rejected.