This paper analyzes the microeconomic behavior of fishers responding to imperfectly enforced regulations through illegal fishing and efforts to avoid detection. An intraseasonal optimization model is analyzed to determine optimal (profit-maximizing) harvesting strategies at the individual fisher level in response to input controls (such as gear or labor usage) or output controls (individual harvest quotas). For each regulatory option, the analysis explores: (a) the manner by which enforcement affects individual decisions concerning fishing and avoidance activity, (b) the level of enforcement necessary to achieve specified conservation goals, and (c) the role of various behavioral parameters in determining fisher decisions. It is shown, in particular, that the nature of avoidance behavior plays a crucial role in determining fisher response to regulations. Broad implications of illegal behavior on the sustainability of fishery systems are also discussed.