Whole Foods is under heavy selling pressure this week after receiving a letter from the Food and Drug Administration about unsanitary conditions at a Massachusetts plant.

The FDA sent the company a warning letter dated June 8 about conditions at a Everett, Massachusetts plant where food products may have been contaminated with "filth or rendered injurious to health." The long list of issues included the presence of Listeria. The FDA also said the company's written response to the conditions was not adequate.

The Austin, Texas, company said that it already corrected the issues and that those steps were not reflected in the FDA's follow-up letter.

Shares are down 1 percent in premarket trading Thursday, and down 10 percent this week.