The alkaline earth silicon nitrides AESiN(2) (AE = Ca, Sr, Ba) are reported, synthesized as clear, colorless, single crystals from molten sodium at 900-1100 degrees C or, in the cases of BaSiN(2) and SrSiN(2), as white powders by reacting powdered intermetallics AESi with flowing anhydrous ammonia at 550-1000 degrees C. Structures were determined from single-crystal X-ray diffraction measurements at 150 K: BaSiN(2) crystallizes in space group Cmca (No. 64) with a = 5.6046(1) A, b = 11.3605(3) A, c = 7.5851(2) A, and Z = 8. The structure consists of pairs of SiN(4) tetrahedra edge-linked to form bow-tie-shaped Si(2)N(6) dimers which share vertexes to form layers and has no analogue in oxide chemistry. SrSiN(2) has a distorted form of this structure (SrSiN(2): space group P2(1)/c (No. 14), a = 5.9750(5) A, b = 7.2826(7) A, c = 5.4969(4) A, beta = 113.496(4) degrees, Z = 4). The structure of CaSiN(2) contains only vertex-sharing SiN(4) tetrahedra, linked to form a three-dimensional stuffed-cristobalite type framework isostructural with KGaO(2) (CaSiN(2): space group Pbca (No. 61), a = 5.1229(3) A, b = 10.2074(6) A, c = 14.8233(9) A, Z = 16).