What is the role of media in nationalist political mobilization? By exposing large masses to common messages, modern media can increase the political salience of group traits and favor collective action along ethnic lines. I study the case of the Basque Country during the late Franco regime and the Spanish democratic transition (1960-1979), where local radio stations operated by pro-nationalist Basque clergy promoted the transmission of regional language and folklore and participation in local civic associations. Using terrain irregularities to isolate random variation in radio signal quality, and a variety of contemporary and archival data on radio programs, political behavior, language use, and victimization in the Spanish civil war, I find that exposure to these ethnic media increased support for new, radical independentist parties tied to ETA. The effect is concentrated in municipalities with lower historical support for Basque nationalism, smaller linguistic group, and a past of Francoist repression. Media can be identity catalysts in communities threatened by cultural assimilation, especially if state violence sustains an ethnic cleavage.