Over the past few years policy makers have mandated that many types of fleet vehicles use clean fuels, such as natural gas, propane, alcohol, or reformulated gasoline, as a means of reducing air pollution. Thousands of fleet operators and millions of dollars could be involved in meeting these Clean Fuel Fleet requirements. This analysis evaluates the costs and emission reductions resulting from the various options available to a variety of fleets operating in the Houston area. The findings of this study are general and should be applicable to other locations implementing Clean Fuel Fleet requirements.The results indicate that fleet operators will incur significant costs to convert their fleets to alternative fuels. The use of reformulated gasoline (RFG), allowed under the Clean Air Act and Texas state laws, but not under the Energy Policy Act, results in the least cost to fleet operators. RFG also provided the most cost-effective emission reductions for most of the fleets and scenarios evaluated.