The total value of the Monkfish (Lophius piscatorius and L. budegassa) production seemed greatly influenced by inter-annual price fluctuations, which could be explained by the evolution of marketing conditions for fish In certain cases, a local market for fish can be defined so that the fishermen are price-makers. In the general case however, producers are subject to external conditions to their local market. This paper addresses the formation of exvessel prices for a single commercial species, Monkfish (Lophius piscatorius and L. budegassa), proposing an identification of market integration for the Monkfish species, considering two major commercial sizes and different types of fishing units. A time series analysis is conducted from 1994 to 2005 on a monthly basis for fishing units exploiting the groundfish communities of the Bay of Biscay and the Celtic Sea. Results show that: (i) price series are globally cointegrated both from the size of species and the size of production units; (ii) the LOP hold for the commercial size 3 and trawlers over 16 meters.