Using microlevel commodity flow data and microgeographic plant-level data, we construct industry-specific ad valorem trucking rate series and measures of geographic concentration to provide evidence on the relationship between transport costs and agglomeration. We find that low-transport-cost industries display significantly more geographic concentration in the cross-sectional dimension and that falling transport costs agglomerate industries in the panel dimension. The effects are large: the fall in trucking rates between 1992 and 2008 implied a 20% increase in geographic concentration on average, all else equal.