Illegal fishing poses a major threat for the conservation of marine resources worldwide. However, there is still limited empirical research that quantifies illegal catch levels. This study uses the Randomized Response Technique to estimate the proportion of divers, and the quantities extracted of illegal "loco" (Concholepas concholepas), a gastropod managed for the past 17 years through a Territorial User Rights for Fisheries system (TURFs) in Chile. Results show that illegal fishing is widespread along the TURFs system, with official reported landings accounting for only 14- 30% of the total loco extraction. Quantitative estimates suggest that ignoring the magnitude of illegal fishing and only considering official landings statistics can mislead conclusions about the status and trends of a TURFs managed fishery. We found evidence of fisher associations authorizing their own members to poach inside TURFs, highlighting the need to design TURFs systems in a way that government agencies and fishers' incentives and objectives are continually adapting to be in line and not at odds. In the same way, government support for enforcement is a key element for the TURFs system to secure the rights that are in place. This study provides insights on how to improve governance of TURFs in Chile and around the world. This article is protected by copyright. All rights reserved.