The Environmental Protection Agency yesterday ventured gingerly into the murky realm of regulating genetically engineered substances, announcing a policy that will require pesticide companies to get the agency's approval before releasing microbial pesticides into the environment.

"We are facing a new generation of pesticides that may -- or may not -- pose special problems in terms of protecting health and the environment," said assistant administrator John A. Moore.

The "interim" policy, which is subject to change, is aimed at a variety of novel pesticides based on naturally occurring or genetically altered microorganisms. The EPA has already registered some natural microorganisms as pesticides, and dozens more are in various stages of development.

But the new products, often touted as safer alternatives to toxic chemicals, can pose hazards of their own. Because they are living creatures and can reproduce, they may spread beyond the fields to which they are applied, causing unforeseen environmental consequences that would be "difficult to control or eradicate," the EPA said.

The EPA's new policy will require companies to give the agency 90 days' notice before conducting any field tests of a microbial pesticide. If the EPA decides there is potential for harm, it may require the company to conduct the experiment under rigid controls or hold off until additional information is available.