This article examines the cost of reducing CO2 emissions in a sample of recently built dry-grind corn ethanol plants. The analysis estimates a translog minimum value function that represents both the minimum cost and the minimum CO2 emissions for given levels of ethanol production. The results indicate that the average plant is able to reduce GHG emissions by 36 percent relative to the level under cost minimization, but production costs are 22 percent higher. The reallocations by which these emissions reductions are achieved are primarily the substitution of wet for dry distillers grains, with the corresponding reduction in the use of natural gas and electricity. To move from least cost to least emissions allocations, ethanol plants would on average produce 25 % more of wet byproduct and 47% less of dry byproduct. Comparing results across observations, the estimated shadow cost of emission abatement ranges from $86 to $190 per ton of CO2, with average value of $124 per ton. This implied shadow cost of abatement can be used as a bench mark for pollution trading and serves to assess the potential response to biofeul regulations.