You still have to wait a few months to play the next major Fallout release for PC and consoles, but publisher Bethesda is already making enough money to fill a postapocalyptic vault from its first crack at mobile.

Fallout Shelter made $5.1 million in its first two weeks on the market, according to industry intelligence firm SuperData Research. That is despite the game only debuting for iOS so far. It should bring in quite a bit more once it rolls out on Android later this year. Bethesda, which held its first ever media event at the Electronic Entertainment Expo trade last month, seized on the opportunity to capitalize on the hype of Fallout 4 by releasing a free-to-play mobile adaptation of the series that places very little pressure on players to spend money, and that has helped it break off a piece of the $30 billion mobile-gaming business for itself.

“In a stroke of marketing brilliance, Bethesda managed to win E3 early by delivering a superb press conference and launching a top-grossing mobile game in its wake,” SuperData chief executive Joost van Dreunen wrote in a report. “Originally developed as a marketing tool for the release of its upcoming Fallout 4, the mobile game is a hit in its own right, passing King’s Candy Crush Saga on the iOS top-grossing charts.”

Fallout Shelter joins other triple-A mobile spinoffs like Call of Duty: Heroes, Halo: Spartan Assault, and Heroes of Dragon Age on iOS and Android. But it also the more successful than those predecessors.

“But more significant than the publisher’s success is the notion that core gaming fans proved to be willing customers for a free-to-play mobile game,” said van Dreunen. “By emphasizing unobtrusive monetization and offline playability, Bethesda managed to earn the respect of a consumer group that is otherwise highly critical of free-to-play monetization. The game’s success further underscores the value of a strong franchise in the otherwise crowded mobile games market.”