First he dissed national treasure Nicole Kidman, who "liked him" and wanted to date the former Saturday Night Live star in 2002, now he's trashed our home-grown delicacy Vegemite on his popular, internationally syndicated television show, The Tonight Show.

The talk show host and his sidekick, comedian Steve Higgins, were encouraged to taste test the savoury spread by some young Australian fans.

"I think Steve should try Vegymite [sic] because he doesn't relly [sic] do anything," Charli, 7, and Jed, 4, wrote in to the host.

Higgins dove in with all the enthusiasm of a hungry Matt Preston by spreading the yeast extract on a suspiciously stale looking piece of bread, or worst, cold toast.

After joking around, quoting Men At Work's Down Under, and confirming he was not a spokesman for the now American-owned condiment, Higgins' bit off more than he could chew before spitting it out.

"I think it's not for the American palate. Apparently French people think root beer tastes weird. I think it [Vegemite] is delicious in Australia," he said before Fallon accepted the taste test challenge.

"I looks like Nutella gone bad. Smells like beer."

"It tastes like beef bouillon that fell on a brewer's floor," Fallon suggested, "Not good."

However in the salty treats defence, the pair made the rookie error in applying the spread as thick as a Kardashian applies make-up.

Vegemite, which is still manufactured in Melbourne but owned by an American confectionary company, is undergoing a PR renaissance, with Cadbury announcing  a Vegemite flavoured chocolate will hits shelves on June 1.

Watch Fallon's contort around the 4:16 mark.