Observations on vertical patterns of migration were made on the Nova Scotian and Gulf of St. Lawrence fishing banks. The commercially important demersal species, cod, haddock, and redfish, were usually closely associated with bottom by day and moved off after dark. While this is the general pattern, variations occurred. Thus, on cloudy or dull days, redfish may remain in mid water. Cod did not always return to the seabed by day. Haddock, usually more closely associated with bottom than the other species, sometimes moved considerable distances from the seabed. Generally, concentrations of fish on bottom by day dispersed in mid water.In accord with vertical patterns of migration, bottom trawl catches were generally the lower by night. There was some suggestion of seasonal changes in the relation between day and night catches of cod and haddock. A suggestion of diurnal variation in size composition was found for haddock and yellowtail. More small haddock were taken by day than by night. Catches of large haddock tended to be the larger at night. Small yellowtail were the more abundant in night tows. No significant diurnal difference was found for large fish.