In February 1937, "Roper" departed California and, after transiting the Panama Canal, joined the Atlantic Fleet. For the remainder of the year, through 1938, and into 1939, she conducted exercises primarily off the mid-Atlantic seaboard and, during part of each year, in the Caribbean. In November 1939, after the outbreak of World War II in Europe, she shifted from Norfolk, Virginia, to Key West, Florida, whence she patrolled the Yucatán Channel and the Florida Straits. In December, she returned to Norfolk. In January 1940, she moved south again, to Charleston, South Carolina, and in March she headed north for duty on the New England Patrol.