Sue, I have lived in Delaware most all my life and as far as I know there has never been a Wilmington in Sussex county. As you observed, present day Wilmington (the largest city in De) is in New Castle County. Kent County separates Sussex and New Castle and as far as I know it has been that way almost since the beginning. In William Penn's day, Delaware was known as the Three Lower Counties. Check out http://nabbhistory.salisbury.edu/County.htms/DelawareHundreds(1782).htmhttp://nabbhistory.salisbury.edu/County.htms/DelawareHundreds(1782).htm It shows the boundries of Delaware Hundreds within the boundries of the individual counties. Hope this helps.