The farmers, who operate Wahl Ranch in Adams County and Dreger Enterprises in Lincoln County, filed the lawsuit in U.S. District Court in Spokane. They are seeking a class-action status, with damages to be determined by the court, The Seattle Times reported.
The plaintiffs said they grow soft white wheat, the same kind of wheat grown by an Oregon farmer who found a genetically modified strain in his field.
Since the genetically modified wheat was found, Japan has stopped ordering soft white wheat from the two Washington farmers.
"We've seen a drop in future and current cash prices on wheat already, and at least one shipment of wheat turned away or refused. We think this is actual damage," said Kim Stephens, a lawyer for the plaintiffs.
Kyle McClain, Monsanto's chief litigation counsel, said the farmers have "scant basis for a lawsuit."
"While we have not yet had the opportunity to review the lawsuit," McClain said, "plaintiff's filing suit now is premature since the facts to date show the report of [Roundup]-tolerant wheat is limited to one field in Oregon, and no such wheat has entered the stream of commerce.
"USDA has said the wheat crop is safe, [the Food and Drug Administration] confirmed food and feed safety in 2004, and USDA has stated repeatedly that there is no indication that glyphosate-tolerant wheat has entered commerce," McClain said.