The paper provides a static analysis of multimarket competition trying to extend classical models of oligopolistic competition including a multimarket effect in firms’ decision problem. After a short definition of what are multimarket oligopolies, we define a multimarket effect as a relation between cross market variables that can be internalised by firms. In case of interrelated costs this will be seen as a sort of externality linked to joint production economies, while in case of independent costs and demands it is modelled as an expected rival cross market reaction. In both cases it modifies competitors’optimal behaviour.A research scholarship from The Bank of Italy is gratefully acknowledged.