ST. LOUIS (AP) - Monsanto Co., the world's biggest seed maker, said it filed a lawsuit against E.I. du Pont de Nemours and Co. and its subsidiary Pioneer Hi-Bred International Inc., claiming unlawful use of Monsanto's proprietary Roundup Ready herbicide tolerant technologies in soybeans and corn.

Monsanto filed suit on Monday in federal court in St. Louis.

The St. Louis-based company said Pioneer publicly spoke of plans to replace Monsanto's Roundup Ready trait with its Optimum GAT trait but recently admitted that its Optimum GAT trait, when used alone, presents unacceptable risks to farmers.

Monsanto went on to claim that Pioneer is misusing Monsanto's Roundup Ready trait to mask these problems, violating Monsanto's contract rights and U.S. patents.

"As the saying goes, imitation is the sincerest form of flattery," said Hugh Grant, Monsanto's chief executive. "However, unlawfully taking technology is neither imitation nor flattery; it is unethical and wrong."

In a statement responding to the lawsuit DuPont group vice president James Borel said this move is another example of Monsanto's use of litigation and "aggressive tactics to preserve their monopoly and attempt to intimidate customers, seed partners and competitors."

DuPont is one of hundreds of seed producers that license Roundup Ready traits and it pays Monsanto more than $200 million a year to use the technology, said Jefferies & Co. analyst Laurence Alexander. The suit may be aimed at pressuring DuPont to increase royalty payments.

"I think it's a market share play," said Paul Baiocchi, senior market strategist at Delta Global Advisors. "Monsanto is trying to discredit DuPont's product in the marketplace."

Monsanto has legitimate concerns about the possibility that DuPont could use of the trait in an unauthorized manner that produces inferior crops, hurting the Roundup Ready brand, according to Soleil Securities analyst Mark Gulley. Monsanto spent $980 million on research and development last year.

Shares of Monsanto rose 85 cents to $89.85 Tuesday. DuPont shares fell 33 cents to $29.07.