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The American-owned Viking River Cruises company tours the Rhine, Main, Elbe, and Danube rivers, with four- to eight-day itineraries that include walking tours at ports of call. The longer cruises (up to 18 days) on the Danube (Donau, in German), which go to the Black Sea and back, are in great demand, so reserve six months in advance. The company normally books American passengers on ships that cater exclusively to Americans. If you prefer to travel on a European ship, specify so when booking. Köln–Düsseldorfer Deutsche Rheinschiffahrt (KD Rhine Line) offers trips of one day or less on the Rhine and Mosel. Between Easter and October there's Rhine service between Köln and Mainz, and between May and October, Mosel service between Koblenz and Cochem. Check the website for special winter tours. Many offer a free trip on your birthday if you bring a document verifying your date of birth.

Cruise Lines

KD Rhine Line. 0221/208–8318; www.k-d.com.

Viking River Cruises. 0800/258–4666; 800/1887–10033; 800/319–6660; www.vikingrivercruises.com.