The crystal structure of the ternary semiconductor IIIa-ZnIn2S4 was investigated using convergent-beam electron diffraction (CBED) and single-crystal x-ray diffraction to elucidate whether the structure is fully ordered, as described by the acentric R3m space group, or has some degree of disorder as described by the centric Rm space group. The CBED technique, based upon dynamical diffraction, permitted the unique identification of the true structural symmetry, while x-ray diffraction could not discriminate between the two space groups. CBED patterns perpendicular to the [001] crystal direction showed the 6mm symmetry characteristic of the Rm space group. This result agrees with early Raman and photoluminescence spectroscopy measurements. The structure is best described by the Rm space group, with cell parameters a = 3.8728(6) Å and c = 37.0664(1) Å and unit-cell volume V = 481.4(1) Å3. The length of the c axis can be described by the expression c = N(3.086±0.003) Å, where N is the number of sulfur layers in the unit cell and Z = N/4 the number of formula units; for our structure N = 12 and Z = 3. The crystal structure consists of a close-packed arrangement of S atoms, with Zn and half of the In atoms distributed in a disorderly fashion in tetrahedral sites, and the other half of the In atoms located in octahedral sites. The atomic positions and an isotropic temperature factor are reported.