The isotopic composition of plutonium in dated Sun Moon Lake sediments was determined by mass spectrometry in effort to trace the source(s) and timing of nuclear fallout in Taiwan. The ²⁴⁰Pu/²³⁹Pu ratios in the sediment core vary by a factor of >5, with the highest ratio, 0.22, corresponding to the maximum level of nuclear fallout in 1963 and the lowest ratios, 0.04-0.06, being identified at two time horizons: 1982-83 and 1992-93. At other depth intervals, the ²⁴⁰Pu/²³⁹Pu ratios fall within 0.15-0.19. The data indicate that global fallout from the stratosphere is the dominant source of Pu in the sediments, where the tropospheric transport of low burn-up debris from Lop Nor in NW China is mostly responsible for the unusually low ²⁴⁰Pu/²³⁹Pu ratios.