This paper analyzes the effects on tax compliance of monitoring the information trails generated by firms’ activities. We exploit quasi-experimental variation generated by a Large Taxpayers Unit (LTU) in Spain, which monitors firms with more than 6 million euros in reported revenue. Firms strategically bunch below this threshold in order to avoid stricter tax enforcement. This response is stronger in sectors where transactions leave more paper trail, implying that monitoring effort and the traceability of information reported by firms are complements. We calculate that there would be substantial welfare gains from extending stricter tax monitoring to smaller businesses.