This study of the offshore distribution of alewife, Alosa pseudoharengus, and blueback herring, A. aestivalis, in the Atlantic Ocean was based on catch data collected over the 16-year period 1963-78 during bottom trawl surveys by the National Marine Fisheries Service and its predecessor agency. All catches of the two species were made where bottom water temperatures ranged from 2° to 17° C, and catches were most frequent at bottom temperatures between 4° and 7° C. Most catches ofboth species were made at stations where depth was less than 100 m. Chi-square analyses indicated that alewives were captured significantly more often than expected in the 56 to no m depth stratum and blueback herring in the 27 to 55 m stratum (P<0.01).During summer and autumn, all catches of the two species were confined to the region north of latitude 40° north in three general areas: Nantucket Shoals, Georges Bank, and the perimeter of the Gulf of Maine (especially in autumn along the northwestern edge of the gulf), Winter catches were between latitude 40° and 43° north, and spring catches were distributed throughout the continental shelf area between Cape Hatteras, N.C., and Nova Scotia. Previous studies on juveniles, food of adults, and differences in time of capture during National Marine Fisheries Service surveys indicated that these species are vertical migrators, apparently following the diel movements of zooplankton in the water column.