Extensive measurements were made of the microwave spectra of nine isotopic species of formyl chloride, HCOCl. Analysis of these spectra gave accurate rotational constants, chlorine nuclear quadrupole coupling constants, and centrifugal distortion constants. The distortion constants, together with the vibrational wavenumbers, were used to evaluate a valence harmonic force field. Effective, substitution, ground-state average, and estimated equilibrium structures are presented.