Industrial organization theory has identified strategic investment in capacity as an important element in competition in oligopolistic markets. In the paper we specify a model of oligopolistic investment behaviour, and test it with panel data for 114 firms in 15 narrowly defined UK manufacturing industries in the 1970s and 1980s. The industries are distinguished according to their market characteristics as fragmented, dominant firm and dominant group sectors. The results indicate behaviour which is noncooperative in fragmented sectors, cooperative in dominant group sectors and competitive in dominant firm sectors.