We have studied the EPR of Mn2+ ions in three samples of cubic nano-ZnS made by different technologies and having different nanoparticle sizes. Manganese occurs in the samples as an uncontrolled impurity. The EPR spectra can be described by two components: the spectrum of the Mn2+ in a cubic environment (Mn2+(C)) with parameters g = 2.0022 +/- 0.0002, A = (-63.5 +/- 0.5).10(-4) cm(-1), b(4)(0) >= 3.5.10(-4) cm(-1); and the spectrum of the Mn2+ ion associated with a planar lattice stacking fault (Mn2+ (F)), with parameters g = 2.0022 +/- 0.0002, A = (-63.5 +/- 0.5).10(-4) cm(-1), b(2)(0) = (-36 +/- 1).10(-4) cm(-1). The ratio of the number of centers Mn2+(C)/Mn2+(F) is 2.1:1 for sample 1 and 1.7:1 for samples 2 and 3. Planar stacking faults are typical lattice faults for cubic nano-ZnS. EPR of the Mn2+ ions lets us monitor the presence of these ions.