This paper uses the Longitudinal Business Database to explore the geographic mobility of three digit SIC industries across cities, and the extent to which this mobility is linked to concentration, scale economies and industry characteristics. We find industries are quite heterogeneous in their degree of mobility, that many industry distributions across cities exhibit a fair degree of churn over time. For example, a city ranked near the top with regard to its share of employment in men’s apparel has an estimated mean first passage time to the bottom of the distribution of seven decades. However, a top ranked steel city would be expected to fall to the bottom in twenty four decades. Surprisingly, industry mobility turns out to be independent of either concentration or scale economies. Our main finding is that technologically advancing industries are more mobile. As new technologies replace older ones, centers of agglomerated industry activity are more likely to change location.

1 The research in this paper was conducted while the authors were Special Sworn Status researchers of the US Census Bureau at the Boston Census Research Data Center (BRDC). Research results and conclusions expressed are those of the authors and do not necessarily reflect the views of the Census Bureau. This paper has been screened to insure that no confidential data are revealed. We are very grateful to Vernon Henderson and Duncan Black for their many contributions, and to Gilles Duranton and Bill Kerr for helpful suggestions. The authors thank participants at the 2004 RSAI and 2005 WRSA conferences. 2 Respectively Brown University, Providence RI, marzaghi@ brown. edu and US Census Bureau, Boston Census Research Data Center, Cambridge, MA jdavis@ nber. org.