Large variations in artesian flow and specific capacity encountered during the installation and testing of a well field completed in the upper Floridan aquifer in Collier County, Florida, are related to differences in dolomite abundance among wells. The wells produce primarily from some thin intervals of fractured crystalline and microsucrosic dolomite that have limited areal extent, as evidenced by a strong boundary effect detected during an aquifer performance test. Microfacies and stable isotopic composition (δ18O = 1.39‰–1.57‰, δ13C = 0.15‰–1.70‰, Peedee belemnite [PDB]) indicate that the dolomite formed by the replacement of marine limestone in marine or brackish pore waters. Dolomite fracturing is likely related to folding that occurred no earlier than the late Miocene. The upper Floridan aquifer behaves as a large-scale dual-porosity system, in that the dolomite intervals with high hydraulic conductivities increase the specific capacities of some wells. The dolomite intervals have little effect on the total volume of water that can be produced from the aquifer, which is controlled by the surrounding lower hydraulic conductivity limestone. The results of this interdisciplinary investigation reveal the high degree of heterogeneity and unpredictability that may occur in carbonate aquifers as the result of diagenesis. Because of uncertainty over fluid-flow paths, heterogeneities in aquifer hydraulics related to diagenesis may greatly impact projects requiring recovery of a specific volume of water, such as pump-and-treat remediation and aquifer storage-and-recovery systems.