To foil would-be terrorists, owners of crop dusters are removing batteries from the planes, wrapping cable locks around the propellers, and in some cases, parking trucks and heavy equipment in front of the aircraft so they can't be moved.

"I haven't talked to anybody in the last few days that isn't making some effort to make sure the airplanes would be hard to steal or misuse in any way," said Dennis Stokes, owner of an Arkansas spraying service. "There's no way they could start one of our airplanes right now."

The government banned flights by crop dusters on Sunday and Monday amid concerns they could be used as weapons of chemical or biological warfare.

Attorney General John Ashcroft told Congress that one suspected hijacker in the attack on the World Trade Center had shown interest in crop-dusters and that another person now in federal custody had downloaded information about the planes from the Internet.

The ban on flights, which was lifted at 12:05 a.m. local time Tuesday, was the second time the planes had been grounded since the Federal Aviation Administration cleared the way for most flights to resume Sept. 14, three days after terrorists flew hijacked airliners into the World Trade Center and the Pentagon.

The National Agricultural Aviation Association, which estimates the ban affects about 3,500 agricultural aviators, posted a message from the FBI on its Web site, urging members to "continue to be vigilant to any suspicious activity relative to the use, training in or acquisition of dangerous chemicals or airborne application" of the chemicals.

Across the South this time of the year, many crop dusters would be in the air almost daily, treating cotton for pests or spraying it with defoliants so it can be harvested.

The crop-dusting business was already struggling because farmers have been using less pesticides. Low crop prices have forced farmers to cut production costs, and the increasing popularity of genetically engineered crops is making the chemicals less necessary. Biotech corn and cotton can produce their own pesticides.

The leading maker of crop dusters, Air Tractor Inc. of Olney, Texas, has cut production in half since 1998. A major competitor filed for bankruptcy protection last year.

The industry "has been bad for three years. This is just killing them with these airplanes grounded," said Tom Wood, publisher of Ag Pilot International, a trade publication.

The one-seater planes, which can cost $250,000 to $1 million, have a range of about 200 miles. The typical plane carries about 500 gallons of chemicals and 200 gallons of fuel.

Pilots say the planes are often difficult to start and even harder to fly. Because the middle wheel of the planes is on the rear instead of the front, as is the case with most modern aircraft, the planes have a tendency to fishtail on takeoff, the pilots say. Flight instructors say investigators have been asking them for information about their students.

"It takes a highly skilled individual to fly one of them. They're very complex," said Pat Kornegay, owner of a south Texas spraying service.

His planes are kept in a hangar that is locked and has an electronic alarm. His home is about 300 yards away. "I don't know what else we can do," he said.

At Southeastern Aerial Crop Service in Fort Pierce, Fla., heavy cable locks have been wrapped around the planes' propellers to prevent them from being started. As further protection, a front-end loader was parked in front of the hangar.

In Florida, pilots will be required to notify state officials of their flight times and aircraft tail numbers.