It was invented in the 1890s to replace tubas in marching bands: in a concert, the tuba is traditionally held on the musician's lap with the bell pointed upward. In marching, however, the musician must hold the tuba in his hands. This proves very difficult due to the weight and shape of the tuba.
The sousaphone is a valved brass instrument with the same tube length as a tuba, but shaped differently so that the bell is above the head, the valves are situated directly in front of the musician a few inches above the waist, and the majority of the weight rests on one shoulder. Thus, the sousaphone can be carried far more easily than a traditional concert tuba, but sounds very similar.
In recent years, sousaphones have been available made of fiberglass instead of brass. The tone is slightly inferior to the brass instrument, but the weight is much less, the cost is lower, and the sound is still acceptable in virtually any marching band performance.
wikipedia.org dumped 2003-03-17 with terodump