Alfalfa-growing areas of the Western United States are often small and widely scattered. Different climatic conditions prevail in different areas. Being able to identify the growing areas and to define those with similar climatic conditions will greatly assist extension and industry workers in decision making regarding alfalfa variety development and variety use recommendations. This bulletin includes a map of the alfalfa growing areas of the Western United States. They are subdivided into 14 climatically distinct zones by using a cluster analysis procedure on 3D-year weather data for 243 weather stations in or near the growing areas. In addition, these 243 locations are ranked in the tables by increasing latitude, longitude, elevation, frost-free days, mean temperature, mean maximum temperature, mean minimum temperature, annual precipitation, and two calculated climatological index values. The information in this bulletin should greatly benefit alfalfa workers in identifying growing areas of the Western United States and understanding their climatological similarities and differences.