Variations in working class political consciousness and behavior are normally ascribed to structural differences between industries. This study, focusing on a single industry, employs a more historical and culturally sensitive approach to explain the emergence of radical and conservative longshore unions on opposite coasts of the United States. Drawing on original oral histories, government and union documents, archival sources and secondary materials, we locate the source of these distinctive political cultures in the differing ideological communities out of which longshoremen on both coasts were recruited prior to World War I. The implications of this finding are briefly discussed.