Abstract: Metamunirite, β-NaVO₃, is found in cavities in sandstone in San Miguel County, Colorado, occurring as fine, fibrous, colourless needles. X-ray powder and precession photographs show the crystals to be orthorhombic, space group Pnma, with a = 14.134(7), b = 3.648(2), c = 5.357(2) Å. They are optically biaxial positive with n_α = 1.780(2) (‖c), n_β = 1.800(2) (‖a), n_γ ≫ 185 (‖b, fibre axis; positive elongation), 2V_z moderate. A crystal structure analysis, confirming the previously determined structure of β-NaVO₃, shows the presence of (VO₃)_n chains made up of zig-zag VO₅ square pyramids. Metamunirite is probably formed by dehydration of munirite, NaVO₃.2H₂O.