In an effort to evaluate the structure of illicit drug markets, two hypotheses are compared. The first hypothesis posits an oligopolistic market composed of a relatively small set of large, hierarchically organized drug distribution networks. The second hypothesis posits a cottage industry of drug trafficking composed of many small groups of traffickers that form and break up easily. Using data collected from Federal, State, and local drug investigators in the Washington-Baltimore area, the authors examined 445 drug trafficking investigations in 1995, 1996, and 1997. Data suggested the cottage industry hypothesis characterized drug trafficking better than the concentrated industry hypothesis. Drug traffickers did not use particularly sophisticated communication technology, and most drug trafficking groups were fragmented and small and had limited connections to other drug traffickers outside the Washington-Baltimore area. Instead of moving large quantities of drugs at any one time, they seemed to move small amounts repeatedly. In addition, drug traffickers did not appear to use extraordinary measures to conceal their packages. The authors conclude by drawing some implications for the control of wholesale drug markets. An appendix notes characteristics of groups most likely to be part of a concentrated drug industry. 25 references, 3 notes, 9 tables, and 5 figures