Economic research on industry location and agglomeration has focused nearly exclusively on manufacturing. This paper shows that services are prominent among the most agglomerated industries, especially at the county level. Because traditional measures of knowledge spillovers, natural resource inputs, and labor pooling explain little of agglomeration in services industries, this paper takes an alternative approach and looks at co-agglomeration to assess why industries cluster together. By considering the location patterns of pairs of industries instead of individual industries, the traditional agglomeration explanations can be measured more richly, and additional measures – like the need to locate near suppliers or customers – can be incorporated. The results show that co-agglomeration between pairs of services industries is driven by knowledge spillovers and the direct trading relationship between the industries, especially at the zip code level. Information technology weakens the need for services industries to co-agglomerate at the state level, perhaps because electronic transport of services outputs lowers the value of longer-distance proximity. These results are in sharp contrast to results for manufacturing, for which labor pooling contributes most to co-agglomeration, and the direct-trading relationship contributes more to state-level co-agglomeration. These differences between services and manufacturing are consistent with simple models of transport costs.