Periodid midwinter, early spring, and summer mortalities of alewives (Alosa pseudoharengus) have been common in the Great Lakes since the first appearance of the silvery marine invader in Lake Ontario in the mid-1870's. In 1967 a nationally publicized dieoff of tremendous magnitude (estimated at several hundred million pounds of fish) in Lake Michigan resulted in losses to industry,municipalities, and recreational interests in excess of $100 million. The cause of these mortalities is still unclear. The apparent inability of this primarily marine species to adjust completely to the Great Lakes has several suspected causes, among which failure to adjust to temperature extremes and fluctuations in the Great Lakes now appears to be of primary importance. Other possible causes are exhaustion of the food supply, failure to osmoregulate (maintain a suitable chemical balance) adequately in fresh water, failure to extract sufficient iodine from the iodine-poor Great Lakes, and a combination of these several possibilities.