We conduct a field experiment to measure cooperation among groups of recreational fishermen at a privately owned fishing facility. Group earnings are greater when group members catch fewer fish. Consistent with classical economic theory, though in contrast to prior results from laboratory experiments, we find no cooperation. A series of additional treatments identifies causes of the difference. We rule out the subject pool and the laboratory setting as potential causes and identify the type of activity involved as the source of the lack of cooperation in our field experiment. When cooperation requires reducing fishing effort, individuals are not cooperative. © 2012 by The University of Chicago. All rights reserved.