The microwave spectra of an isoxazole-CO complex and four of its isotopomers have been measured between 7 and 18 GHz with a pulsed nozzle Fourier transform microwave spectrometer. Rotational constants, centrifugal distortion constants, and N-14 quadrupole coupling constants have been fitted to the measured transition frequencies of each isotopomer. The permanent electric dipole moment has been determined from Stark effect splittings. A unique structure has been found for the complex by taking both the moments of inertia of the isotopomers and the quadrupole splittings into account. The complex is planar, with CO lying approximately radially away from nitrogen in the isoxazole ring.