Dielectric and ultrasound measurements of berthierite FeSb 2 S 4 reported in the literature revealed a phase transition at T c ؍ 348 K. In the present study the phase transition was con-5rmed by MoK ssbauer spectra exhibiting anomalous behavior close to 350 K. Precise lattice parameters measured in the temperature region 305+380 K show a change of slope at about 350 K. The crystal structure of FeSb 2 S 4 has been determined using single-crystal X-ray di4raction data at both room temperature and 370 K above the phase transition. The crystal structure is orthorhombic, Z ؍ 4. At 295 K a ؍ 11.412(2) A s , b ؍ 3.763(1) A s , c ؍ 14.161(3) A s , and R ؍ 0.0280, and at 370 K a ؍ 11.421(2) A s , b ؍ 3.771(1) A s , c ؍ 14.173(3) A s , and R ؍ 0.0274. No symmetry change has been observed. At both temperatures the space group of FeSb 2 S 4 is Pnma with small changes in atomic parameters and interatomic distances. The transition observed at 348 K is, therefore, to be classi5ed as a weak phase transition going on without essential changes in the crystal structure.