We investigate inter-industry variation in the relative incidence of advertisers' utilization of in-house rather than independent advertising agencies. To account for this variability, we develop a set of hypotheses drawing. The first perspective emphasizes scale economies and "double marginalization." The second perspective is that of transactions cost economics. The hypotheses are tested using a database consisting of a cross section of 70 two digit SIC industries measured at two points in time, 1991 and 1999.