The No. 1 seller of Costco-brand products online isn't Costco

Amazon Marketplace - not Costco - is the top online seller of Costco's private-label brand, Kirkland Signature, according to a new study.

Amazon is responsible for 70% of Kirkland's online sales, while Costco is only responsible for 23%, according to the study by analytics firm 1010data.

Jet.com is in third place with a 5.5% share of Kirkland's online sales.

"I have always associated the Kirkland Signature brand with Costco, which is why I was so surprised to learn that Costco.com is NOT the No. 1 seller of Kirkland items online," a 1010data researcher wrote.

Prices on Kirkland's products vary between all three websites.

For example. the average selling price for a 200-count box of Kirkland kitchen trash bags is $19.01 on Costco.com, $22.72 on Amazon Marketplace, and $14.25 on Jet.com, the study found.

And not all items that are available on Amazon Marketplace, such as Kirkland Signature olive oil, can be purchased at Costco.com.

Customers must have a Costco membership to shop in the warehouse chain's stores. Membership fees start at $55 annually.

Here are the top-selling Kirkland items by units sold on each website:

Disclosure: Jeff Bezos is an investor in Business Insider through hispersonal investment company Bezos Expeditions.