Since 1884, when commercial fishing first started on Lake of the Woods, the commercial catch and the fish populations have changed markedly. Lake sturgeon (Acipenser fulvescens) which was originally the most important commercial fish is now almost extinct. The lake whitefish (Coregonus clupeaformis) has also shown a striking decline. Intensive commercial fishing was an important factor in the decline of these species. The practical disappearance of other species which were not subject to heavy fishing, the longnose sucker (Catostomus catostomus), goldeye (Amphiodon alosoides), and quillback (Carpiodes cyprinus), however, suggest some basic change in habitat. Destruction of the spawning beds in the Rainy River by pulp-mill wastes was probably the most important of these habitat changes. A correlation was found between the strength of year classes of saugers (Stizostedion canadense) and June rainfall. Recent declines in the population of yellow pikeperch (Stizostedion vitreum) do not appear to be caused by pollution since its effects on the habitat were evident long before declines appeared in the pikeperch catch. Saugers and yellow perch (Perca flavescens) seem to be on the increase. These species compete with the pikeperch for food. During the summer months, the saugers usually occupy deeper waters than the pikeperch but not consistently enough to preclude competition. The present fishing methods are selective and favor the survival of saugers and yellow perch which are not being taken in proportion to their abundance nor being exploited as intensively as the pikeperch. It is recommended that fishing methods be adjusted to permit greater utilization of yellow perch and saugers.