How does something change but stay the same? For certain musical groups it's quite easy. The musicians tend to gain in technical and compositional expertise while holding on to the playing style that first fired their imagination. The result is often a sound that steadily improves while remaining static.

Such is the case with Natural Bridge. The Washington-based group, which appeared last night at the Cellar Door, began two years ago as a fusion ensemble and persists in pursuing that style. They rely heavily on the hackneyed fusion formula of thumping bass lines, pleasingly innocuous melodies and jerking, thrusting rhythms.

All of which is regrettable, because the musicians of Natural Bridge have become a formidable musical force. Keyboardist Gregg Karukas, once a fine craftsman, has evolved into a daring and imaginative soloist, coaxing a variety of surprising sounds from his synthesizer. And new bassist Mike Mannering is the most agile exponent of his instrument in town -- he plays with a skillful abandon that plucks at both the mind and the feet.

The members of Natural Bridge have the potential to fashion a sound that is truly their own. When they stop looking to their past, they could have a most promising future.