Castolon, also known as La Harmonia Ranch and Campo Santa Helena, was a small community in southwestern Texas, United States, located in what is now Big Bend National Park along the Rio Grande.

The location was first settled in 1901 by Cipriano Hernandez, who farmed the area and built the original Castolon Store, now known as the Alvino House.

The Castolon area was the site on a U.S. Army encampment, called Campo Santa Helena, to house units patrolling the Mexican–American border.

In 1921 the La Harmonia Company was established in the barracks, operating a trading post and farming cotton.

A sudden wind direction change with strong gusts "showered embers across the Castolon Historic District, igniting the latrine building and barracks (which housed the La Harmonia Store and Visitor Center)".