The crystal structure of a biogenic aragonite from the nacre of an ammonite shell was obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinement. The well-preserved ammonite sample is from Alberta, Canada, and is from the Cretaceous period. The aragonite structure was refined in space groupPmcn, Z = 4, and the cell parameters obtained are a = 4.96265(2), b = 7.97016(4), c = 5.74474(3) Å, and V = 227.222(2) ų. The chemical analyses indicate a formula of [Ca_0.995Sr_0.004Ba_0.001]_∑=1.0(CO₃). The average <Ca–O> and <C–O> distances are 2.5281(3) and 1.2871(6) Å, respectively, and the average <O–C–O> angle is 119.94(8)°. The CO₃ groups are non-planar. Based on crystal-structure data for biogenic and non-biogenic aragonite samples, aragonite from ammonite nacre has minimal structural distortions and is very similar to non-biogenic aragonite, in particular, a sample from Spain.