In this paper the central issue is to quantify economic and environmental consequences for dairy farms when converting to biological dairy farming. A linear programming model is used to model a typical extensive and intensive dairy farm in The Netherlands. From the results it appears that extensive farms benefit from conversion while intensive farms lose income. Environmental consequences are quite diverse. The nitrogen surplus after conversion is much lower than before while ammonia emission is higher after conversion due to a higher number of animals. On the extensive farm the phosphate surplus is much higher after conversion due to the fact that a shortage of nitrogen in the biological situation can only be made up by applying animal manure (slurry) from other farms with consequential overfertilization of phosphate. When environmental legislation is introduced, biological farms appear to lose more income than conventional farms.