Vugs in lower Carboniferous limestone form Abercriban, southern Wales, are filled with calcite crystals displaying chemical growth zonation. Nine samples, physically separated from individual growth zones amalgamated from different crystals and analyzed by using standard acid-digestion techniques, yielded ranges in δ13C and δ18O of +0.4‰ to -2.6‰ and -6.3‰ to -9.0‰, respectively. There is a general trend toward lighter carbon- and oxygen-isotope compositions in the younger zones. Thirty laser-ablation analyses of individual ∼30-μm-wide zones from the same material revealed a much wider variation in δ13C and δ18O (+3.8‰ to -3.0‰ and -3.1‰ to-11.O‰), though with similar mean compositions. Significant jumps in isotopic composition were revealed between consecutive growth zones. These jumps are too large (<1‰ in δ13C, <3‰ in δ18O) to be explained by temperature effects related solely to sediment loading and unloading. The high spatial resolution of laser sampling has identified a fine-scale isotopic zonation that has hitherto gone undetected. This zonation suggests that calcite precipitation may be punctuated (or even driven) by changes in nature of pore water and sources of solutes.