We have investigated cathodoluminescence (CL) and decay dynamics of photoexcited states in CdSe/ZnSe single-fractional monolayer structures grown by molecular beam epitaxy on a GaAs(0 0 1) substrate. CL spectra measured at 10 K by scanning the focused electron beam with a diameter of 200 nm show sharp peak structures superimposed on a broad spectrum. The observed spectra are ascribed to exciton luminescence from individual CdSe-based dots in two di!erent lateral size ranges: large dots (15}40 nm) and small ones (6}10 nm). Decay behaviors of photoluminescence measured in the picosecond time region have been interpreted in terms of exciton migration in the nanostruc-tures. 2000 Elsevier Science B.V. All rights reserved.