Bethesda's free-to-play app Fallout Shelter is currently the second-highest grossing app on mobile beating even King's Candy Crush Saga, which was earning around $2.55m (£1.62m) a day during the first quarter of 2015.
Released on iOS immediately after Bethesda's E3 press conference in the early hours of 15 June, Fallout Shelter is a habitat simulator in which players can build, customise and maintain their own post-apocalyptic vault.
Resources used to keep your vault's inhabitants happy in the game are limited, which is where the ability for players to purchase additional resources if they choose comes in.
Speaking to Gamesbeat during E3, Pete Hines, vice president of Bethesda's PR and marketing, said: "We're very pleased. Fallout Shelter is doing amazingly well, with no real build-up.
"To take such a light approach to monetisation compared to other free-to-play games... we felt like we wanted to err on the side of caution and make it very unobtrusive, make sure it was fun first and foremost. So far so good."
Fallout Shelter was announced on-stage during Bethesda's conference. It is a surprise stopgap for Fallout fans before fully fledged console title Fallout 4 is released on PS4, Xbox One and PC in November.
"Having Fallout Shelter... they were expecting a lot of stuff, but they're not expecting us to have an app and have it available right now," Hines said. "We debated that. There's a reason people do soft launches, to get to a larger audience and make sure everything holds up. There's a risk in not doing that. But the fun of saying it's available right now – that was too good to pass up."
Fallout Shelter is currently available to download on iOS with an Android release promised soon.