Sales of organic and natural products are soaring — but you would never know it from the share price of Whole Foods Market, the premier purveyor of such merchandise.

Shares are almost 50 percent lower than they were in February, the high point of the year.

Investment analysts are almost uniformly negative on the company, worried that competition from mainstream retailers, which are increasing the amount of organic and natural items in their mix, will impede the growth of Whole Foods.

Costco, for instance, claims to be the biggest seller of organic foods, and Walmart now sells Wild Oats, a brand of organic products, at the same price as similar conventional brands.

“Conventional retailers can get it into their stores more cheaply, and they can be more predatory on pricing,” said Mark Retzloff, a pioneer of the natural and organic foods retail business. “If one of those stores is just down the street from a Whole Foods, there’s a big segment of their customer base that isn’t going to shop at Whole Foods anymore.”