Abstract The X-ray crystal structure of 5-methyl-5, 10-dihydrophenarsazine, C 13 H 12 NAs, has been determined. The crystals are orthorhombic, space group P2 1 2 1 2 1, a= 8.511 (4), b= 5.690 (1), c= 23.535 (9) Å, V= 1139.7 (1.0) A 3, and Z= 4. Least-squares refinement converged to R= 0.049 and Rw= 0.045. The central ring is in a boat conformation and the 5- methyl group is in the axial position with respect to the central ring. The folding angle ...