The collection of delinquent fines is a vast and ongoing public administration challenge. In the United Kingdom, unpaid fines amount to more than 500 million pounds. Managing non-compliant accounts and dispatching bailiffs to collect fines in person is costly. This paper reports the results of a large randomized controlled trial, led by the UK Cabinet Office’s Behavioural Insights Team, which was designed to test the effectiveness of mobile phone text messaging as an alternative method of inducing people to pay their outstanding fines. An adaptive trial design was used, first to test the effectiveness of text messaging against no treatment and then to test the relative effectiveness of alternative messages. Text messages, which are relatively inexpensive, are found to significantly increase average payment of delinquent fines. We found text messages to be especially effective when they address the recipient by name.