The British Columbia halibut fishery provides a natural experiment of the effects of ´privatizing the commons". Using data from the fishery two years before private harvesting rights were introduced, the year they were implemented and three years afterwards, a stochastic frontier is estimated to test for changes in technical, allocative, scale and economic efficiency. The study shows that privatization coincided with a significant increase in some short-run measures of efficiency. Despite these improvements, overall the fishing fleet still remains well below the best practice frontier. The results indicate that even short-run gains in efficiency from privatization may take several years to materialize and that pre-existing regulations can have significant effects on changes in efficiency. An analysis of the changes in the fishery shows that only by paying careful attention to the divisibility, exclusivity, transferability, duration, quality of title and flexibility of the property rights will the potential gains from the privatizing of common-pool resources be realized.