Physical and chemical characteristics of four dimictic eutrophic lakes of northcentral Minnesota with marl deposits are discussed in relation to production of invertebrate animals (benthos and plankton) that are the basic food of fish . Two of the lakes have a history as productive fish lakes and the other two as problem fish lakes. The productive lakes, as a type, have a larger surface area, a longer shallow-water littoral shelf, and a lower proportion of marly soils in the littoral zone than do the problem lakes. The standing crop of invertebrates was about twice as great per habitat unit in the productive as in the problem lakes. Surface waters of the productive lakes were somewhat higher in total phosphorus, total nitrogen and total iron than in the problem lakes. Since the productive lakes have brownish or greenish water as opposed to clear water in the problem lakes, chelation of iron and other trace metals by organic compounds also may be involved in productivity