Abstract.-Stunting in populations of bluegill Lepomis macrochirus is a major management problem in the upper midwestern United States. Traditionally, stunting has been attributed to a lack of food resources. An alternative theory suggests that stunting may result from harvest of the large parental males that inhibit spawning by smaller males, allowing small males to direct energy to reproduction instead of to growth. We used a population model to simulate restrictions on harvest of large males under various conditions of vulnerability to angling, growth rate, and angler effort. Regulations tested were a daily bag of 5 fish and seasonal bags of 50 fish or 50 fish but only 1 trophy specimen (>180 mm total length); a limit of only 1 trophy fish during the spawning season; and a spawning season closure. Growth and angler effort had the greatest effects on size structure in simulated bluegill populations; high growth and low effort led to notable increases in mean fish length. Tested regulations and male vulnerability to angling had little effect on size structure although spawning season closure produced a modest increase in mean length. The simulations also suggested that increases in the proportion of large fish in bluegill populations require regulations that reduce total angler effort. Furthermore, to be successful, effort regulations must be applied to populations that have the potential for good growth.