Use of open wells to conduct aquifer tests in fractured crystalline rock aquifers is potentially problematic, because open wells can hydraulically connect highly permeable fracture zones at different depths within the rock. Because of this effect, it is questionable whether estimates of the hydraulic properties of the rock obtained from analysis of an open-well aquifer test in fractured crystalline rock are representative of the true rock properties. To address this issue, rock properties estimated from such a test are compared with rock properties estimated from an aquifer test at the same location in which packers were installed in wells to restrict vertical fluid movement along the wellbores. Results indicate that when the open-hole test is analyzed with a numerical model that accurately represents the three-dimensional heterogeneity structure in the wellfield, the open-hole test estimates of the rock hydraulic properties are similar to the packer-isolated test estimates. When the open-well test is analyzed using a model having an oversimplified representation of the rock heterogeneity, the estimates of rock transmissivity are similar to the transmissivity estimates from the packer-isolated test, but the storativity estimates are larger than the storativity estimates from the packer-isolated test. These results indicate the importance of accurately representing the rock heterogeneity structure when analyzing an open-hole aquifer test in fractured crystalline rock.Use of open wells to conduct aquifer tests in fractured crystalline rock aquifers is potentially problematic, because open wells can hydraulically connect highly permeable fracture zones at different depths within the rock. Because of this effect, it is questionable whether estimates of the hydraulic properties of the rock obtained from analysis of an open-well aquifer test in fractured crystalline rock are representative of the true rock properties. To address this issue, rock properties estimated from such a test are compared with rock properties estimated from an aquifer test at the same location in which packers were installed in wells to restrict vertical fluid movement along the wellbores. Results indicate that when the open-hole test is analyzed with a numerical model that accurately represents the three-dimensional heterogeneity structure in the wellfield, the open-hole test estimates of the rock hydraulic properties are similar to the packer-isolated test estimates. When the open-well test is analyzed using a model having an oversimplified representation of the rock heterogeneity, the estimates of rock transmissivity are similar to the transmissivity estimates from the packer-isolated test, but the storativity estimates are larger than the storativity estimates from the packer-isolated test. These results indicate the importance of accurately representing the rock heterogeneity structure when analyzing an open-hole aquifer test in fractured crystalline rock.