Also "Evolva" might qualify, but I haven't played it.

Well, Warhammer 40,000: Dawn of War II – Retribution has the zerg's bigger borthers as playable factions: the Tyranids

System shock 2's The many, they're pretty biological

The Fallout games?

It might be worth checking out the old RTT based on Starship Troopers. While not an outstanding game, it was decent enough (you don't get to play as the bugs though, not to my knowledge at least). Do avoid the FPS though, it is terrible.

If you can find it really cheep, the old game Warbreeds might be worth a look. It's not good, but it is interesting

I recommend Conquest: Frontier Wars (unless you run Windows 8), it has a lot of similarities with Starcraft.

I have and I love it. It's not the most graphically breathtaking game on the planet but its a lot of fun.

Another vote for SMAC.

A biological faction? Fancy some bacteria and virii?

[url=http://en.wikipedia.org/wiki/Genesis_Rising:_The_Universal_Crusade]Genesis Rising[/url]

Bioengineered spaceships :)

Oh, and Entomprph lets you morph into an alien insect. - And you meet lots of others of these critters.

This. Absolutely this.

Also, in Endless Space, you can play Amoeba.

Scanning the threads, when I first read this, I read Games with Biological Functions. Thank God this isn't a thread about Sim Toilets!

+1
Made me laugh.

I'm reading this discussion with interest, as Zerg were probably the main thing for me that made Starcraft so interesting. I loved both playing them, and playing against them. Ah, seeing your brown fungus grow around the area you are claiming... or vice versa, seeing the Zerg fungus slowly retract as you destroy the filthy Zerg installations.