This paper considers an oligopoly where …rms produce a joint and indivisible environmental harm as a by-product of their output. We …rst analyze the e¤ects on the equilibrium of alternative designs in environmental liability law, secondly, we discuss the rationale for "non-conventional" competition policies, i.e. more concerned with public interest such as the preservation of human health or environment. We study …rms decisions of care and output under various liability regimes (strict liability vs negligence) associated with alternative damages apportionment rules (per capita vs market share rule), and in some cases with damages multipliers. We …nd that basing an environmental liability law on the combination of strict liability, the per capita rule, and an "optimal" damages multiplier, is consistent with a conservative competition policy, focused on consumers surplus, since, weakening …rms' market power also increases aggregate expenditures in environment preservation and social welfare. In contrast, a shift to the market share rule, or to a negligence regime, may be consistent with a restriction of competition, since …rms'entry may instead lead to a decrease in aggregate environmental expenditures and losses of social welfare. Nevertheless the …ne tuning of the policy requires speci…c information from a Competition Authority, which we discuss as well.