Bioeconomics to date has focused almost exclusively on lumped parameter models where the resource is described by a single state variable defined as biomass. While such models are convenient mathematically they may be inappropriate in fisheries where recruitment and fecundity are dependent on the age structure of the resource. This paper develops a reasonably general multiple cohort model and derives conditions for optimal harvest and age structure based on a discrete time control problem which maximizes the present value of net revenues subject to recruitment and spawning constraints. The model is applied to the hard clam resource in Great south Bay which is located on Ling Island, New York The steady state optimum calls for exclusive harvesting of the younger, ( and more valuable), "littleneck" cohorts; leaving the older, (and less valuable), "cherrystone" and "chowder" cohorts to specialize in regeneration.