We model a three firm oligopoly (the Spanish fertilizer industry from 1976 to 1988) which was subject to price regulation in the form of price ceilings. A theoretical and econometric model is developed in order to identify simultaneously the behaviour of the firms and the degree to which regulation constrained the price. The collusive market-sharing arrangements involving asymmetric firms, and the less collusive outcomes of Cournot and Stackelberg, together with their constrained counterparts, are considered as particular behavioural hypotheses. The estimation of an aggregated and a disaggregated version of the model, by GMM techniques, leads us to identify the regulation-constrained Stackelberg equilibrium as the most likely outcome given the observed data.