We compare the effects of an emission tax, and those of an emission standard, on welfare and pollution levels under oligopolistic market structures. We consider the cases where the number of firms is fixed and where there is free entry and exit of firms. When the number of firms is fixed (i) an emission standard is welfare-superior to a pollution-equivalent emission tax, and (ii) an emission tax is emission-superior to welfare-equivalent emission standard. Under free entry and exit, the results are just the opposite when the inverse demand function is concave.