This paper analyses the impact of terrorism on tax enforcement policies by focusing on the case of the Basque Country. The presence of externalities in tax administration attributable to the costs of terrorism is investigated by undertaking a theoretical analysis. The findings of this are tested using Spanish data extracted from repeated surveys and other sources. By employing ordered response models, evidence is found of the negative impact of terrorism on tax enforcement as it is perceived by residents in the Basque Country and Navarre. In particular, this impact is found to be stronger for entrepreneurs and liberal professionals. No significant impact is found for individuals resident in the rest of Spain.