The rotational spectrum of sulfur choride fluoride, ClSF, has been observed for the first time in the frequency range 8-26 GHz by means of a pulsed molecular jet Fourier-transform microwave spectrometer. The unstable sample molecule has been prepared using a pulsed electrical discharge in jets containing a mixture of SF6 and SCl2 in Ne. Besides the parent species (ClSF)-Cl-35-S-32-F-19, the isotopomers (ClSF)-Cl-37-S-32-F-19 and (ClSF)-Cl-35-S-34-F-19 could be observed in natural abundance. Rotational constants and quartic centrifugal distortion constants as well as nuclear quadrupole coupling constants due to Cl-35 and Cl-37 and spin-rotation constants due to Cl-35, Cl-37, and F-19 are given. The data were used for the determination of r(0), r(Delta P), r(s) structural parameters. Additionally, the new data were used for the refinement of the molecular harmonic force field. Results from those harmonic force field calculations were applied in the evaluation of the ground-state average structure, r(z), and the estimation of the equilibrium structure, r(e). The r(s) structure is r(SF)=160.653(162) pm, r(SCl)=199.437(65) pm, and angle(ClSF)=100.732(81)degrees. The diagonal elements of the Cl-35/Cl-37 quadrupole coupling tensors have been obtained, and are interpreted in terms of the bonding at Cl-35/Cl-37. Negative F-19 spin-rotation constants suggest a close analogy of the electronic structures of CISF and SF2.