The introduction of Iceland's current ITQ system in 1991 has been followed by significant consolidation of the harvesting sector. This paper quantifies the rate of consolidation by measuring the concentration of quota holdings at company and vessel level for eight commercially important demersal and pelagic species over the period 1991-2017, using concentration ratios and the Herfindahl-Hirschman index. These metrics are complemented by calculation of the Instability Index, which is a measure of competitive rivalry. The results indicate a significant increase in concentration across all species at both company and vessel level, although company consolidation appeared to accelerate relative to fleet consolidation during the early 2000 ' s when the Icelandic financial sector was privatised and then slowed in the wake of the financial crisis in 2008. The Instability Index fell for all species, indicating a general decline in competitive rivalry. Concentration varies greatly by species, and is lowest for cod and haddock which are caught extensively by small and medium-sized vessels and highest in the pelagic sector, reaching levels associated with market power. Finally, there is evidence that regulatory limits on individual company quota holdings may be starting to hamper further consolidation and any associated efficiency gains.