Soil bacteria that were genetically altered and then released into a wheat field have been found not to move very far, a Clemson University scientist reported at last week's meeting in Boston of the American Association for the Advancement of Science.

The finding was from the first experiment designed solely to track the dispersal of genetically engineered microbes released into the environment. Critics have argued that released altered bacteria might spread far and cause ecological harm.

The bacteria, a common soil species called Pseudomonas fluorescens, carried no potentially harmful genes and had only been modified by the addition of innocuous marker genes that made it easy to track them and their progeny and to distinguish them from their natural relatives already in the soil. The Monsanto Co. developed the engineered bacteria.

The test began last November when winter wheat was planted in South Carolina. At weekly intervals, the bacteria were checked. At first, they multiplied prodigiously to colonize the wheat roots. Then their numbers dwindled. After 10 weeks, they were about 1 percent as numerous as at their peak.

The microorganisms stayed close to their roots. At the most, they spread seven inches horizontally and 12 inches vertically. Eventually, however, the bacteria that roamed farthest died out.

"I was surprised. Even after rain and snow, they just stay right there around the roots," said Ellis Kline, a Clemson microbiologist doing in the experiment.

The experiment will run 18 months, through the harvest of this crop, a normal follow-on crop of soybeans and the harvest of a second wheat planting.