Fluorescence properties have been studied for Mn:ZnS crystallites with average diameter of 4 nm prepared by an aqueous colloidal method under 266 nm light excitation. The intensity ratio of the blue band at similar to 430 nm to the orange band at similar to 590 nm has decreased after the preparation on a time scale of hours in aqueous solution. On the other hand, hyperfine structures of Mn2+ in the electron paramagnetic resonance spectrum have increased markedly on the same time scale in solution samples. These phenomena are attributed. to the redistribution of defect centers in nanocrystals. Such phenomena have not been observed in samples incorporated into poly(vinyl alcohol). The orange emission is mainly due to the (6)A(1) <-- T-4(1) transition of Mn2+, while the blue emission is tentatively assigned to the donor-acceptor pair transition in which the acceptor is related to the Zn2+ vacancy. Fluorescence decay times of the blue and orange bands have been found to be similar to 10 ns and similar to 1 ms, respectively, the latter being the same as in the bulk samples. A weak fluorescent component with fast kinetics observed in the orange region has been identified as a tail of the blue band, No lifetime shortening of the Mn2+ emission due to quantum confinement has been observed, contrary to reports in the literature.