Parallel determinations of carbon-14 and ionium dates on a stalagmite from the Cango Caves provide evidence of variations in the carbon-14 content of the atmosphere beyond the range of the California tree-ring sequence. During the Holocene growth period, the thorium-230 dates are compatible with the tree-ring calibrated carbon-14 dates. At 18,000 BP and between 30,000 and 40,000 BP, the carbon-14 ages are markedly younger than the thorium-230 ages, suggesting that the carbon-14 level of the atmosphere was considerably higher at these times. Between the thorium-230 ages of 35,000 and 29,000 BP, the carbon-14 ages remain nearly constant at 29, 500 BP, indicating that carbon-14 production must have been drastically reduced during this period. The carbon-14 fluctuation is greater than that predicted by Barbetti (1980) but it may be explained by postulating a substantial increase in the geomagnetic dipole field, for which there is mounting evidence. -- AATA