Immigration records at the National Archives contain more than just information about passengers traveling into the United States from foreign ports. Quite often they also include accompanying lists of crewmen, both American and foreign-born, who worked on the vessels. These lists exist because of an early 19th-century law that required the masters of American vessels leaving U.S. ports on foreign voyages, or arriving at U.S. ports from abroad, to file crew lists with the customs agent at their port of entry. The original purpose of the act was to provide a convenient means of identification to protect American crews against the threat of impressment. Foreign vessels were initially exempt from the law, but by the early 20th-century the Immigration Act of 1917 required specific documentation about all alien seamen on vessels entering the U.S.
19th- and 20th-century crew lists are available textually and on microfilm (the filmed records constitute the only existing copies we have of these records; after the Immigration and Naturalization Service microfilmed the records in the 1950s they destroyed the originals). Vessel crew lists from many Atlantic, Gulf, and Pacific coast ports are described on the Archival Research Catalog (ARC), such as lists for the port of Philadelphia from 1793 to 1901 (ARC ID 573978). To find the records on ARC, use the search term “crew lists”.