Sartorite, PbAs2S4, has been examined by transmission electron microscopy. The mineral has a monoclinic (but metrically orthorhombic) subcell with a = 19.62, b = 7.89, c = 4.19 Å, β = 90°, and space group P21/n. In addition to the strong substructure reflections, G, corresponding to this monoclinic subcell, it also exhibits satellite reflections (in general incommensurate with respect to the subcell reflections) at G ± mq, where m is an integer and q, the primary modulation wave vector ~6/13 (101)*. Some variation in the direction of the primary modulation wave vector was observed with the direction of the modulation being rotated up to 3° away from (101)* toward (001)*. Crystals with twin-related super-lattices were also observed. Superlattice formation in sartorite is believed to result primarily from modulations in the lengths of arsenic sulfide chains within the structure.