This study looks at the factors that influence the development of marketing co-operation among cluster-based firms It examines data from SMEs operating within the salmon farming industry in two different regions Scotland and Chile Analyses indicate that informal social networks help explain the observed relationship between geographical proximity and inter-firm marketing co-operation, especially for firms located in peripheral rural communities A theoretical model is proposed for further research in the field that, until recently, has been traditionally analysed only by economists Practical implications are suggested for practitioners and policymakers (C) 2010 Elsevier Ltd All rights reserved