The iOS release of Fallout Shelter pulled in more than $5m in its first two weeks, according to Superdata's latest monthly report on digital sales figures.
The company credits the spin-off's success to Bethesda pulling off the minor miracle of getting core gamers to step into and stick around for a free-to-play mobile experience.
"While we've seen mobile spin-offs based on AAA game franchises before--Call of Duty: Heroes, Halo: Spartan Assault and Heroes of Dragon Age--Bethesda managed to maximize exposure and reap the benefits," Superdata CEO Joost van Dreunen said.
"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. 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."
Superdata collects digital point-of-sale data from developers, publishers and payment service providers each month, and June proved to be a surprisingly strong period. According to the firm, US digital game revenue was up 18 per cent year-on-year to $1.02bn – thanks in part to digital console and PC game sales, in addition to solid numbers for mobile.
"Generally speaking, Western engagement with mobile games often falls during the summer," van Dreunen stated. "But last month's mobile revenue was up 4 percent over May. Mobile revenues were up 20 per cent compared to June 2014 and reached $367m."
The largest digital segment – Free-to-play social, mobile, and MMO games – grew 13.5 per cent as well, bringing in revenue just shy of $660m.