Three paramagnetic centers associated with manganese substituting for Ti4+ have been observed at 300 K by EPR in reduced crystals of SrTiO3. The first is a Mn2+ ion having cubic symmetry, with isotropic g and A values given by g=2.0036±0.0005 and A=(82.6±0.1)×10-4 cm-1. The second center is characterized by an axially symmetric EPR spectrum with effective g values of ge≅2.00 and ge≅5.9 at X band. The spectrum is attributed to ΔM=±1 transitions between the Sz=±12 levels of a Mn2+ ion located in a strong tetragonal crystalline field. The tetragonal field presumably arises from a nearest-neighbor oxygen vacancy, hence we designate it as Mn2+-Vo, in analogy with the previously investigated Fe3+-Vo center. An exact computer diagonalization of the S=52 matrix, together with data on a second fine-structure transition, yielded a zero-field splitting parameter |2D′′|=1.088±0.0010 cm-1, and g values of g=2.0030±0.0005 and g=2.0082±0.0060. The hyperfine-structure splitting parameters were determined to be A=(76±1)×10-4 cm-1 and B=(65±1)×10-4 cm-1. The normally forbidden Δm=±1 transitions were also observed for this center, and their separations are consistent with an S=52 formalism. The third paramagnetic center also gives rise to a highly-anisotropic spectrum with effective g values of ge=7.945±0.001 and ge<0.4, and a hyperfine constant A=(37.3±0.1)×10-4 cm-1. The center is assigned to a Mn3+ ion associated with a nearest-neighbor oxygen vacancy, hence we designate it as Mn3+-Vo. The one fine-structure transition observed is assigned to occur between the M=±2 levels of Mn3+(3d4,S=2). By careful measurements at X- and K-band frequencies, the cubic zero-field splitting between these levels was determined to be a=0.054±0.001 cm-1. © 1977 The American Physical Society.