Two paramagnetic defect centers associated with phosphorous impurities and corresponding to S=½ and I=½ have been observed in irradiated single-crystal calcite. One center, which is produced by γ irradiation, has an ESR absorption spectrum which consists of six doublets at 77°K and below, each exhibiting orthorhombic symmetry with spin-Hamiltonian parameters given by gxx=2.0072, gyy=2.0033, gzz=2.0122, Axx=52.53 Mc/sec, Ayy=56.16 Mc/sec, and Azz=52.21 Mc/sec, where the relation between the x, y, z coordinates and those of calcite is discussed. As the temperature is raised, the lines of this spectrum are observed to broaden and at about 150°K the spectrum collapses into one doublet pattern which exhibits symmetry about the calcite (111) direction suggesting that the coordinates of the defect center undergo some form of motion whose mean z axis is parallel to the calcite (111) direction. This behavior is attributed to thermally induced reorientations of the defect between six magnetically inequivalent sites. The activation energy for this reorientation is estimated to be 0.10 eV from low-temperature line-broadening data. An electron—nucleus double-resonance absorption spectrum observed at 4.2°K has demonstrated the doublet to be associated with 31P. It is concluded that the center giving rise to this ESR spectrum is the PO42− molecule—ion. The second defect center is produced by neutron irradiation and has an ESR absorption spectrum consisting of a doublet pattern which exhibits symmetry about the calcite (111) direction. This center is stable up to about 300°K and at 77°K has spin-Hamiltonian parameters given by g∥=2.00165, g⊥=2.00162, A∥=2293.2 Mc/sec, A⊥=1789.6 Mc/sec, and gnβn/gβ=−0.000608. This spectrum is identified with the PO32− molecule—ion which is believed to occur as a substitution for the carbonate ion.