We use confidential, U.S. Census Bureau, plant-level data to investigate aggregation in external reporting. We compare firms’ plant-level data to their published segment reports, conducting our tests by grouping a firm’s plants that share the same four-digit SIC code into a “pseudo-segment.” We then determine whether that pseudo-segment is disclosed as an external segment, or whether it is subsumed into a different business unit for external reporting purposes. We find pseudo-segments are more likely to be aggregated within a line-of-business segment when the agency and proprietary costs of separately reporting the pseudo-segment are higher and when firm and pseudo-segment characteristics allow for more discretion in the application of segment reporting rules. For firms reporting multiple external segments, aggregation of pseudo-segments is driven by both agency and proprietary costs. However, for firms reporting a single external segment, we find no evidence of an agency cost motive for aggregation.