As I play the game, I notice that eventually tougher enemies show up. From what I've read, it sounds like this is based on elapsed time rather than "research checkpoints", but I haven't seen any specifics. What kind of time frames are we talking about for various enemy types to appear? Are there any fixed enemy events that happen after a certain number of days? Does difficulty level affect this?

I want to have an idea of when I can expect so that I can better plan my strategy.

  • 2
    I think its based on both. The elapsed time thing is to make it so you can't build up your resources to a degree where the next stage becomes too easy, and the research checkpoints makes it so you can't rush through the game using only starter rifles. I think it was mentioned that alien progression was more 'aggressive' in higher difficulties (was in some game preview or something). Knowing the exact checkpoints for each phase would be a lot of help.
    – l I
    Commented Oct 11, 2012 at 18:20
  • I'd like to know so I know where I am objective-wise vs science vs soldier skills. I know I just downed the overseer ship but only after that and downing an abductor ship did I get my first psionic soldier. The overseer didn't appear until right after I got my first firestorm too. So it might be based off science too.
    – Shykin
    Commented Oct 11, 2012 at 18:37

3 Answers 3


It seems that some enemies are tied to some missions, while others will appear anyway, but it seems that if you are quick enough they will appear earlier after certain missions.

Credit to yx and Sterno in the comments below !

Enemies which are tied to missions

  • Outsiders - First appear in a downed UFO. One appears in most (but not all) UFOs after that.

  • Sectoid commanders - first appears during the assault on the alien base. After this point, Sectoid commanders appear on the bridge of alien UFOs, replacing the Outsider. After the overseer ship mission is complete, they are replaced by Ethereals. Sectoid Commanders may also appear on harder alien abduction missions, usually in small groups. Source

  • Ethereals - First appear in the assault purple ufo mission, and then randomly in missions afterwards

Enemies which will appear anyway

  • Sectoids
  • Floaters and Heavy Floaters
  • Mutons and Elite mutons
  • Berserkers
  • Sectopods
  • Thin men

Spartacus mentioned that Chrysalids may be tied to the first terror mission.

  • 1
    I think all the 'ship bosses', i.e. outsider, sectoid commander and ethereal are tied to the story. However, I've seen the other units show up randomly. For example, my first attempt on classic I didn't run into sectopods until the overseer UFO mission. Second time around I ran into them pre overseer UFO (I intentionally delayed my game a bit though to prepare more).
    – l I
    Commented Oct 18, 2012 at 14:54
  • 2
    I still haven't shot down the overseer UFO, and I've seen floaters, elite mutons, berserkers, sectopods, etc. Pretty sure I've seen everything but Ethereals at this point. Sectoid commander's didn't seem to show up until after the alien base though (even though everything else had been out for a while... I've had a very slow-moving game)
    – Sterno
    Commented Oct 18, 2012 at 16:16
  • 1
    I don't know for sure, but chrysalids (which you left out) may be tied to the first terror mission. You also forgot to list drones (which can appear by themselves much earlier than cyberdiscs and sectopods)
    – l I
    Commented Oct 18, 2012 at 18:07
  • 2
    Updated with what I know about Outsiders and Commanders. Haven't run into Ethereals yet so didn't add info about that one. Would still like to know if there's a general timeline for how many months have to pass for mutons to appear, then the elite versions, etc.
    – Sterno
    Commented Oct 18, 2012 at 19:36
  • 1
    I think the outsiders will continue to appear until you capture them for research.
    – jonleech
    Commented Nov 30, 2012 at 2:22

In my experience enemy races are tied to elapsed time and the two key missions: Alien Base and Overseer. They are not tied to game level.

March: Expect Sectoids, Thin Men and Outsiders.

April: Add Floaters and Chrysalids. If you reach the Alien Base, add Drones and Sectoid Commander.

May: Add Mutons.

June: Add Cyberdisks and Heavy Floaters. If you reach the Overseer, add Muton Elite, Sectopods and Etheral.

July: Add Muton Beserker.

  • 1
    Note that the Slingshot DLC missions cause some enemies to appear earlier: Thin Men and Chrysalids on the first one, Muttons on the second and Cyberdisks on the third one IIRC
    – Zommuter
    Commented Sep 15, 2013 at 11:14

The alien commanders are mission-based:

  • Outsiders appear during ship missions (ufo landing and crash sites) until the player completes the Alien Base mission.

  • Sectoid Commanders are first encountered during the Alien Base mission. They replace Outsiders during ship missions.

  • Etherals are first encountered during the Overseer crash site mission. They replace Sectoid Commanders during ship missions.

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