Hilario Urbizu San Roman has been wanted in Spain since 1994 for his alleged participation in the killing of six Civil Guards in northern Spain in 1980.
He was spotted in the crowd attending a bullfight Saturday in the central city of Aguascalientes.
The fugitive had gone to watch the celebrated bullfighter José Tomás at a much anticipated comeback corrida. At the same ring five years ago Tomas almost lost his life after being gored.
Many Spaniards fled to Mexico during and after the Spanish civil war of 1936-39, and the country has also been considered a haven for members of Eta.
The paramilitary group has been fighting for an independent Basque homeland in northern Spain and southern France since the 1960s. It is blamed for more than 800 deaths.
Spain's Ministry of the Interior announced that he had been caught on the twitter account.
The Basque separtist group renounced violence in 2011 but refuses to dissolve and turn over its arms as demanded by the government of Madrid and Paris.