Rio Rancho, a part of the Albuquerque Metorpolitan Statistical Area, is the largest city of Sandoval County. A small portion of the city extends into northern Bernalillo County. It is the third-largest and fastest-growing city in New Mexico. As of the 2000 census, the city population was 51,765. It has risen to 75,978 as of the 2007 census estimate. Currently there are three zip codes assigned to Rio Rancho, 87124 (51,404 pop.), 87144 (37,719 pop.), and 87174 (Post Office Box).