The European plaice (Pleuronectes platessa) is a commercial species with stock management advice given through the estimation of fish length at maturity (L50) (Mahé et al., 2007). Estimating the maturity of fish species has been worked on by the International Council for the Exploration of the Sea (ICES) that wants to harmonize the definitions, terminologies and practices used to determine the different maturity phases. Project MATO (MATurité Objective des poissons par l’histologie quantitative) will, among other things, bring knowledge on the ovarian histology of the plaice in order to correlate histological maturity with macroscopic parameters like the size color and texture of the ovaries. With this knowledge, the aim of this study is to set up a protocol that will help define the different maturity phases of Pleuronectes platessa in a completely objective way while following the ICES (2018) maturity scale.