Genetic variation within a species is determined by natural selection, recombinant, genetic drift and mutation. Gene flow acts against genetic differentiation. It is not clear to what extent the gobies Pomatoschistus minutus and P. lozanoi differentiate genetically on the macroscale (the European continental shelf) and on the microscale (the Belgian continental shelf), as well in time as in space. Allozyme and mitochondial DNA markers will be used to estimate the genetic structure and changes in this structure.