In most real-world commons, the exact resource quantity is rarely known, and the rate at which the resource regenerates is not always predictable. Annual salmon runs, for example, are based on breeding rates and many other factors; those fishing never know exactly how many fish will be available for next year's season. The present study employed a computerized commons dilemma simulation to investigate the impact of uncertainty in pool size and regeneration rate on individuals harvest choice and the efficiency with which groups managed the resource pool. Both types of uncertainty produced significant declines in individual restraint and group efficiency, although the group-level effects tended to be stronger than those at the individual level. Implications for the management of real-world commons are discussed.