 The US Post Office was established April 10, 1878, with Noah W. Bernard as the first Postmaster; still in operation at ZIP code 85601.Freighter and rancher Pedro Aguirre established a stage stop in Arivaca and the Buenos Aires Ranch.In 1879 he built the historic Arivaca Schoolhouse, which was listed on the National Register of Historic Places in 2012, as the oldest standing school building in Arizona.