Abstract.A single crystal of natrolite, Na2A12Si3Olo.2H20, was studied by X-ray diffraction methods at room temperature. The intensities were measured with MoK~ radiation (2=0.7107/~) in a complete sphere of reflection up to sin 0/2 = 0.903 A-1. The structure was refined in the orthorhombic space group Fdd2 with a= 18.2929(7)/~, b= 18.6407(9)/~, c=6.5871(6) ~, V = 2246/~3, Z = 8. A refinement of high-order diffraction data yielded reliability factors of R(F)=0.9%, Rw(F) = 0.8%, GoF= 1.40 for 1856 high-angle reflections (0.7 _ sin 0/2 < 0.903 /~ -1) and R(F)=I.0%, Rw(F ) = 1.2%, GoF= 3.07 for all 3471 independent reflections in the complete sphere of reflection. The T-O distances as well as the T-O-T angles were found to be strongly influenced by the different bond strengths received by the individual oxygen atoms. The T-O distances calculated using Baur's extended valence rule agree on average within 0.003 fk with the observed values. Art analysis of the mean square displacement amplitudes allowed a separation of the external and internal vibrational amplitudes along the T-O bonds as well as along the Na-O and H20-O bond directions and the calculation of force constants. The internal vibrational amplitudes (A U) of the T-O vibrations are in the range of 5 to 11 x 10 -4 ,~2, that is about one order of magnitude smaller than the mean square displacement amplitudes of the external vibrations. The corresponding force constants are F= 354 to 824 Nm-a. The values of the force constants of the motion of the Na-ion and the water molecule against the framework oxygen atoms lie in the range between F = 57 and 293 Nm-1. This is the first instance where displacement amplitudes from a zeolite structure refinement could be apportioned between contributions from internal and external vibrations for individual bonds.