The authors present a theory of involuntary unemployment which explains why the unemployed workers ("outsiders" ) are unable or unwilling to find jobs even though they are prepared to work for less than the prevailing wages of incumbent workers ("in siders"). The outsiders do not underbid the insiders since, were the y to do so, the insiders would withdraw cooperation from them, making their work unpleasant with harassment, and thereby reducing the prod uctivity and increasing the reservation wages of the underbidders. Th e resulting labor-turnover costs create economic rent which the insid ers tap in wage setting and, as a result, involuntary unemployment ma y arise. Copyright 1988 by American Economic Association.