The best growth days of Amazon's (AMZN) - Get Report Prime juggernaut could be over, for now.

Amazon Prime penetration among U.S. households may have peaked, according to a new survey from RBC Capital Markets analyst Mark Mahaney. Over the last five years, Amazon Prime penetration in the U.S. has surged from 25% in June 2013 to 55% in May 2018. May's result was unchanged compared to June 2017. 

It marked the first time in Mahaney's four years of surveys on Amazon that the penetration number hasn't grown. The latest survey reflects data from 2,000 respondents.   

"This is a bit of a surprising result, especially given that Amazon recently reported 100 million Prime subscribers globally," says Mahaney. "This implies Amazon's Prime membership expansion is likely driven by growth in international markets."

Adds Mahaney, "We see Prime's value proposition as compelling, but Amazon may have to boost its Prime offering (more video, music, & faster shipping). This also raises questions about Prime pricing power." Mahaney still thinks Prime presents good value for consumers despite a recent price hike, putting membership on par with a McDonald's (MCD) - Get Report Happy Meal and a Netflix (NFLX) - Get Report monthly subscription.

But if Prime penetration has plateaued in the near-term, it could be reason to lighten up on Amazon's red-hot stock. Shares have surged 44% year to date on bets Amazon will only get more dominant across many sectors.  

Uh-oh, Amazon bulls.

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