A Guinness shortage ahead of the festive season in December prompted a chain reaction: pubs were scrambling, drinkers were upset, and Diageo was left to cope with the outsized demand.

Guinness’s rival is having a moment as the supply shortage continues into January. 

Murphy’s, an Irish stout made by Dutch brewer Heineken, has seen demand skyrocket by 632% in the U.K. in December compared to the same period in 2023, according to data shared with Fortune by Heineken.   

“These outstanding sales figures go to show demand for stout among U.K. pub-goers is in no way slowing down, while an increasing number are eager to discover stouts outside beyond the market leader,” a Murphy’s U.K. spokesperson said. 

Guinness is the market leader in Irish stout, holding a sizable 80% market share. It’s also become wildly popular across the Atlantic, where Gen Z has been chasing the drink once considered niche. 

The Guinness fever has been influenced by celebrities like Kim Kardashian, who have been seen with the drink, while content creators have dedicated videos to finding the best Guinness pint. That’s contributed to double-digit growth for Guinness in Europe and beyond. 

“It was always, culturally, a huge part of Ireland, but now the 18-25 year old has been massively impacted. Young people are drinking Guinness because it’s all over social media,” Daragh Curran, who runs a YouTube page called “Guinness Guru,” told Fortune last year.

Murphy’s, meanwhile, has been a much smaller player. Although it’s been around since 1856, Heineken only purchased it in 1983. Heineken also sells a slew of other beers in the U.K. such as Amstel and Birra Moretti. 

But as Diageo continues deciphering how to keep the Guinness supply flowing, Murphy’s has swooped in to seize the moment. 

“Amid reports that pubs were running out of the ‘black stuff’ ahead of the festive period, HEINEKEN UK took to the street of London to hand deliver kegs of Murphy’s,” in time for the company’s Christmas campaign, the statement said. (“Black stuff” is an indirect reference to Guinness, which has a deep brown color and a bitter taste.)

Murphy’s has also created a map of all the pubs in the U.K. where its stouts are in stock—over 500 pubs in total. 

Guinness offers a similar feature for its original stout and the non-alcoholic variant. 

Diageo has a problem that beer brands would kill to have: demand outstripping supply. The company is investing in expanding its breweries to eventually meet people’s thirst for stout.

“Demand for Guinness remains at unprecedented levels. We are making good progress in replenishing stocks across the distribution network throughout January to meet ongoing consumer demand," a Diageo spokesperson told Fortune.

This story was originally featured on Fortune.com