Consumers wanting to avoid genetically engineered salmon, if it eventually reaches grocery stores, might have a hard time being sure. That is because the Food and Drug Administration said on Thursday that the salmon would not have to be labeled as genetically engineered.

That is consistent with the F.D.A. stance on the widely eaten foods made from genetically modified corn, soybeans and other crops. The F.D.A. on Thursday rejected two petitions from groups asking for required labeling of genetically engineered foods.

Agency officials explained on Thursday that the law required labeling of “material” aspects of food, and that use of genetic engineering per se is not material. A significant change in the nutritional content of a food would be an example of a material change, and that altered nutritional profile would have to be on the label, but not the fact that it was produced by genetic engineering. (In the case of the salmon, the agency said there were no material differences between the genetically engineered salmon and a conventional counterpart.)

Still, the F.D.A. on Thursday issued draft guidance for voluntarily labeling salmon and final guidance for voluntarily labeling foods made from bioengineered crops.