This paper examines the amount and organization (individual vs. collective) of lobbying by firms in administrative agencies. It explores the power and limitations of the collective-action theories and transaction-cost theories in explaining lobbying. It introduces a dataset of over 900 lobbying contacts covering 101 issues at the Federal Communications Commission (FCC) in early 1998. It finds that the structure and conduct of large-firm lobbying at the FCC is consistent with the predictions of theories of transaction costs and the main results of theories of collective action. Small firms show little sensitivity to collective-action issues or transaction-cost issues in the organization of their lobbying, but they do lobby less when having to reveal proprietary information. In sum, large firms behave in a manner largely consistent with theoretical predictions, while small firms do not.