Except forming two Cs-rich minerals like pollucite and londonite, cesium generally occurs in trace amounts in potassium-bearing minerals owing to its very low Clarke value and large ionic radius. However, in the Koktokay#3 pegmatite (Altai, NW China), lepidolite is extremely enriched in cesium (typically 21–26 wt% Cs₂O). Cs-enriched lepidolite is restricted to the inner of the pegmatite, where four types of occurrence are characterized by using in situ techniques (EMP: electron microprobe, micro-XRD: micro-area X-ray diffractometer and Raman probe: micro-area Raman spectrometer) as: (1) outer zones on Cs-poor lepidolite cores; (2) fine overgrowths on muscovite veinlets; (3) veinlets in Cs-poor lepidolite and (4) veinlets in elbaite. There is an inverse correlation between K and Cs and in terms of ^VIAl versus Cs/(K+Cs), the Cs-dominant micas are an analog of the Li-rich mica polylithionite. Micro-XRD patterns and micro-Raman spectra indicate that Cs-dominant polylithionite structurally corresponds to minerals of the lepidolite series. According to its distribution and compositional characteristics, the Cs-dominant polylithionite seems to have formed at the magmatic to hydrothermal transition stage of pegmatitic magma evolution as a result of marginal replacement of early-formed lepidolite by Cs-rich fluids. Alternatively, it may have formed through direct precipitation from Cs-rich fluids. The results of the present study have important implications for the storage of nuclear waste in that Li mica such as polylithionite is a good candidate for immobilizing high-level radioactive cesium waste.