The dynamics of the Lake Michigan deep chlorophyll layer (DCL) were studied from the period of late spring isothermal mixing (May) through mid-stratification (July-August) in 1982–1984. After the onset of thermal stratification, the DCL developed in the 15–30 m region and deepened to 25–50 m in July and 40–70 m in August. Chlorophyll and phytoplankton carbon concentrations in the DCL averaged, respectively, 1.80X and 1.34X epilimnetic concentrations during early stratification (June). Those factors increased to 5.70X and 2.60X during mid-stratification. Although phytoplankton carbon concentrations within the DCL changed on average only 31 % from May through July-August, phytoplankton species composition exhibited pronounced shifts. Measured phytoplankton growth, sedimentation, and zooplankton grazing rates suggest DCL formation was attributable to in situ growth and to a lesser extent to sedimentation and shade adaptation. By July, sedimentation resulted in a net loss from the DCL. With the deepening of the DCL during mid-stratification, the importance of in situ growth decreased while the importance shade adaptation increased. In situ growth was only important in the upper part of the DCL. Zooplankton grazing increased during mid-stratification and was at least partially responsible for phytoplankton concentrations decreases in the 20–50 m region.