This paper pursues a research agenda inspired by Richard Nelson and Sidney Winter's Evolutionary Theory of Economic Change (1982). This seminal work applied the Darwinian concepts of variation, replication and selection to the evolution of firms. It proposed a level of evolution, replication and selection at a level higher than individuals or genes, involving the replication and selection of routines and institutions. Significantly, the applicability or otherwise of these Darwinian concepts depends on precise definitions of terms such as replication and selection. The present essay builds on previous work where the concepts of replication (Godfrey-Smith, 2000; Aunger, 2002; Hodgson, 2003b) and selection (Price, 1995; Frank, 1998; Knudsen, 2002b, 2003) have been refined. We deploy the key concepts of 'replicator' and 'interactor' from the modern philosophy of biology (Hull, 1981, 1988). It is shown that while habits and routines can be regarded as replicators, there is a case for regarding firms and similarly cohesive organizations as interactors. We explore some of the implications of this result and provide an important component in the construction of a multiple-level evolutionary theory, involving replicating units at several socio-economic levels.