International Journal of Industrial Organization 19 (2001) 493–518 www.elsevier.com / locate / econbase Multiproduct activity in an experimental differentiated oligopoly * ´ ´ Aurora Garcıa-Gallego, Nikolaos Georgantzıs ´ ´ Departamento de Economıa, Campus Riu Sec, Universitat Jaume I, 12071-Castellon, Spain Received 1 March 1998; received in revised form 1 March 1999; accepted 1 July 1999 Abstract In a symmetric differentiated experimental oligopoly with multiproduct firms we test the predictive power of the corresponding Bertrand-Nash equilibria. Subjects are not informed on the specification of the underlying demand model. In the presence of intense multiproduct activity, and provided that a parallel pricing rule is imposed to multiproduct firms, strategies tend to confirm the non-cooperative multiproduct solution.  2001 Elsevier Science B.V. All rights reserved. Keywords: Multiproduct firms; Multiproduct Bertrand-Nash equilibrium; Collusion; Product differen- tiation, Price parallelism JEL classification: C72; C9; L1 1. Introduction Oligopoly theory has associated the existence of multiproduct firms in a market 1 with a broad spectrum of phenomena such as product proliferation and predation , * Corresponding author. Tel.: 134-964-728-590. ´ E-mail address: georgant@eco.uji.es (N. Georgantzıs). 1 ´ Despite the result in Martınez-Giralt and Neven (1988), according to which the introduction of a second variety by the same firm is never profitable, Judd (1985) and Aron (1993) offer examples of product proliferation by multiproduct firms wishing to preempt differentiated product markets. In our paper, industry configurations are exogenously given. In terms of the aforementioned literature, this would imply that each multiproduct firm prefers being the one who offers an extra variety – if it has the opportunity to do so – rather than allowing another firm to sell it. 0167-7187 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0167-7187(99)00027-2