Fluid pressures up to 1.7 times greater than hydrostatic have been measured in argillaceous Paleozoic rocks of low permeability in southern Ontario and western New York State. These supernormal formation fluid pressures were measured at depths of 50–310 m using submersible pressure transducers with straddle packers and multiple-packer casings isolating the test intervals. Measurements were obtained over periods of 7–46 months following casing installations. The pressure measurements from 11 monitoring wells are compiled and supporting hydrogeologic data for 5 selected wells are used as examples to illustrate the occurrence of supernormal fluid pressures in the Ordovician, Silurian, and Devonian sedimentary sequence of southern Ontario and western New York State. Possible explanations for the occurrence of supernormal fluid pressures in sedimentary rock are evaluated considering the available geologic and hydrogeologic information obtained from the monitoring wells. Based on this review, it is hypothesized that gas migration and accumulation from deeper distant sources via permeable vertical pathways is the most plausible explanation for the observed fluid pressures, although secondary contributions from local neotectonic activity are also possible. The implications of such supernormal fluid pressures on regional groundwater flow in sedimentary rocks and related activities such as waste disposal in sedimentary rock are briefly discussed.Key words: supernormal fluid pressure, sedimentary rocks, gas migration.