The pure rotational spectra of PCl3 and AsCl3 have been observed in the microwave region, and the observed fine structure has been partially explained. The distance P–Cl is 2.043±0.003A and the angle Cl–P–Cl is 100° 6′±20′. The distance As–Cl is 2.161±0.004A and the angle Cl–As–Cl is 98° 25′±30′.