Natural gas samples from wells located in Ordovician and Cambrian strata of southwestern Ontario have been characterized isotopically and compositionally. Together with data from Silurian natural gas wells (Barker and Pollack, 1984), these results provide important constraints on gas formation and migration within the Lower Paleozoic strata of southwestern Ontario. Compositionally these gases are ＇＇wet＇＇, composed predominantly of CH4 (57-92% by volume), with significant concentrations of higher hydrocarbons (C2+ = 5-33%) and N2 (1-16%). Notably, gases in the Ordovician (Black River Group) and Cambrian strata that form the contact rock with the Precambrian basement have very high He concentrations (up to 1.2 % by volume) compared to concentrations of < 0.15% in overlying strata. CH4 isotopic results indicate that the gases are thermogenic in origin, with a significant bacterial component (10-45%) in several Silurian reservoirs. Isotopic and compositional thermal maturity indicators support the conclusion that the gases originated in mature to overmature source rocks. Such high maturation levels are inconsistent with in situ gas production in the immature to marginally mature southwestern Ontario strata, and suggest that gases migrated into southwestern Ontario from more mature source rocks outside the region. The distribution of estimated gas formation temperatures (based on carbon isotope geothermometers) indicates a source in the Appalachian Basin to the southeast of the study area. Migration pathways over hundreds of kilometres appear to be controlled by the Cambrian strata and their contact with the Algonquin Arch. He-3/He-4 ratios for gases from Cambrian wells range from 3 x 10(-8) to > 8x10(-8) (R/Ra = 0.02-0.06) - values typical of radiogenically produced crustal He. The highest He-4 concentrations and He-3/He-4 ratios are consistently found in the Cambrian and Ordovician wells located close to the Precambrian basement. An observed correlation between He-4 concentrations and He-3/He-4 ratios may reflect a mixing trend between in situ gas and a deeper end-member enriched in both He-4 and He-3/He-4. At present, it is not possible to determine whether this end-member originates in the underlying Precambrian basement, or like the hydrocarbon component, originates in the Appalachian Basin and has migrated into southwestern Ontario following pathways controlled by Cambrian strata and the Algonquin Arch.