A lawsuit seeking to be certified as a class action has been filed on behalf of consumers in New York and California against the owner of Quaker Oats after testing found traces of the pesticide glyphosate in some oatmeal.

While the level of glyphosate, the active ingredient in the popular weed killer Roundup, detected in the oatmeal falls well below the limit set by federal regulators for human consumption, the lawsuit accuses Quaker of false advertising because it markets the oatmeal as “100% natural.”

Quaker, which is owned by PepsiCo, says on its website that the oats used in its products are grown in an environmentally responsible way. “Since oats require less herbicide spray than many other grains, there is less risk of pollutants and groundwater contamination,” it says.

The lawsuit, filed in Federal District Courts in New York and California, contends that such statements are false and misleading. “There is nothing unlawful about Quaker Oats’ growing and processing methods,” the suit says. “What is unlawful is Quaker’s claim that Quaker Oats is something that it is not in order to capitalize on growing consumer demand for healthful, natural products.”

In a statement, the Quaker Oats Company said that it did not add glyphosate during any part of the milling process but that it might be applied by farmers to certain grains before harvest.

The company said it puts the oats it receives through a cleansing process. “Any levels of glyphosate that may remain are trace amounts and significantly below any limits which have been set by the E.P.A. as safe for human consumption,” the company said.

A test paid for by lawyers for the plaintiffs, the Richman Law Group, found glyphosate at a level of 1.18 parts per million in a sample of Quaker Oats Quick 1-Minute. This is roughly 4 percent of the 30 parts per million that the Environmental Protection Agency allows in cereal grains.

An independent lab in California did the testing, using liquid chromatography-mass spectrometry, a technique widely used by medical and chemical labs and pharmaceutical research.

Although testing was done only on Quick 1-Minute, the lawsuit also makes claims against Quaker Oats Old-Fashioned and Quaker Steel Cut Oats.

Glyphosate is one of the most widely used pesticides around the world. The Monsanto Company began selling the pesticide in the 1970s. But it was the introduction of genetically engineered crops two decades later that fueled the sharp increase in the use of the pesticide. Those crops now account for most of the corn, soy, sugar beets and canola grown in the United States.

Glyphosate also is widely used in home gardens, on golf courses and other places.

Last year, the World Health Organization classified glyphosate as a “probable” carcinogen. Since then, environmental and consumer groups have begun to focus their efforts on the pesticide, and a growing number of reports on glyphosate in food have surfaced.

Earlier this year, the Food and Drug Administration said it planned to begin testing some foods for glyphosate.

The biotech industry and food companies contend that the levels of glyphosate found in products fall well below limits set by government officials. The European Union sets the average daily level of intake for glyphosate at 0.3 milligrams per kilogram, while the Environmental Protection Agency sets it at 1.75 milligrams per kilogram daily.

Kim Richman, the lead lawyer of the firm representing the plaintiffs, said the amount of glyphosate was not the issue. “The issue is that Quaker advertises these products as 100 percent natural, and glyphosate in any amount is not natural,” he said.

Oats are not a genetically engineered crop. But glyphosate is increasingly being used as a “dessicant” to dry out crops to speed harvesting.

The lawsuit was filed over the weekend in New York and California, and, Mr. Richman said, would be filed in other states this week.

The plaintiffs are seeking refunds for purchasers. They also are asking that PepsiCo either be required to reformulate the products or disclose the presence of glyphosate in them.

Correction: May 2, 2016

Because of an editing error, an earlier version of this article omitted the given name of the lead lawyer of the Richman Law Group. He is Kim Richman.

Because of an editing error, an earlier version of this article omitted the given name of the lead lawyer of the Richman Law Group. He is Kim Richman.

A version of this article appears in print on May 2, 2016, on page B4 of the New York edition with the headline: Traces of Pesticide Belie Quaker Oats’ Claim It Is 100% Natural, Lawsuit Asserts.

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