The aim of this paper is to analyze whether the two main species of the fishery of the Grand Sole (monkfish and hake) constitute a horizontally integrated single market in the products' space. A cointegration approach is used based on monthly price data of first sale for the period 2003-2010. The main results show that the markets of both species are cointegrated and therefore, constitute a single market. There is no bidirectional causality and hake is the leading species.