Amazon's deal for supermarket chain Whole Foods has sent US shares in grocery stores, big retailers and food makers and distributors plunging.
It is rare for a single deal to have a big effect on the broader stock market but Amazon's agreement to buy Whole Foods Market did.
Investors wondered if Amazon will do to grocery stores and supermarkets what it has done to sellers of goods like clothing and office supplies - force them to make big changes or be supplanted.
Neil Saunders, managing director of the research firm Global Data Retail, said Amazon is likely to push supermarkets and grocery stores to slash prices, which will affect the companies that make and distribute those products.
"As Amazon enters the grocery market proper, it will put a lot more pressure on existing grocers," he said. "Those grocers will respond by cutting prices and that will cut profits for the distributors."
Elsewhere, energy companies rose as oil futures bounced back from their lowest price this year and utilities and industrial and basic materials ground out modest gains.
Thanks to a late gain, the Standard & Poor's 500 index inched up 0.69 points to 2,433.15.
The Dow Jones industrial average added 24.38 points, or 0.1%, to a record high of 21,384.28.
The Nasdaq composite fell 13.74 points, or 0.2%, to 6,151.76. The Russell 2000 index of smaller company stocks shed 3.36 points, or 0.2%, to 1,406.73.
Online juggernaut Amazon announced it was acquiring Whole Foods in an all-cash deal worth 13.7 billion US dollars (£10.7 billion).
Many investors had expected Amazon to get into the grocery business. It already runs AmazonFresh, a grocery delivery service for members of its Prime service, and it recently opened a few grocery stores.
Investors dumped retailers, drug stores, and even discount chains. Many of them have started trying to sell more groceries in the last few years to try to capitalise on shoppers' desire for fresher, more natural food.
Amazon is a unique threat to many retailers because it does not mind losing money for long stretches. The company might be able to sell cheap groceries as it makes its money from its cloud computing business and its gigantic online marketplace.
"Is the future of grocery store shopping going to be a point and click experience, or is it going to be going to a grocery store?" said Dan Morgan, senior portfolio manager at Synovus Trust.