Agar.io[a] is a massive multiplayer online action game created by Brazilian developer Matheus Valadares.

Largely due to word of mouth on social networks, it was a quick success, becoming one of the most popular browser and mobile games in its first year.

[5] The browser version currently holds five game modes: FFA (Free-For-All), Battle Royale, Teams, Experimental, and Party.

The mobile version of the game includes Classic (like FFA), Rush Mode, Burst, and Battle Royale.

In the experimental mode, feeding a virus will push it instead of englarging it, making it easier to split other players.

[12] He thought about adding more complex biological features like photosynthesis and mitochondria, but ultimately decided to strip the game down to its current simple mechanics.

[13] Valadares never marketed Agar.io outside of his single post on 4chan, where he received feedback and users formed "cartels" to get on the in-game leaderboard.

[13] Its initial success is instead attributed to its popularity among online content creators such as YouTubers and Twitch streamers.

[15] Commentators also suggested Agar.io's accessibility as a free browser game, as well as the addictiveness of its simple and intuitive mechanics, as reasons for its unexpected success.

[5] After meeting with him in Lisbon, mobile game publisher Miniclip began working with Valadares at the end of April 2015.

They were attracted by Agar.io's wide appeal; the game already had five million daily players and Miniclip executive producer Jamie Cason stated that their staff were all playing it within a week.

[22][23] Game journalists also noticed that the ability to choose usernames and skins enabled players to declare their support for geopolitical causes and figures in-game.

[19][24] One reviewer jokingly recounted: "I've ... seen the Earth be swallowed by Pubes, Steam absorb EA, and France split in two and flee from Mars before being eaten by Your Mom's Ass (which was, quite frankly, enormous).

It noted that Agar.io provided a platform for competing ideologies in a "survival of the fittest" simulation when they would instead be shut down on social media.

The game soon reached the top 10 most downloaded apps on iOS and Android, buoyed by the similar word-of-mouth and attention from YouTubers that had propelled Agar.io.

Characterized by simple graphics and gameplay in a free for all multiplayer arena, .io games received around 192 million visits in 2017.