[After discovering the alien bleeds acid.]
Parker: It's got wonderful defense mechanism: you don't dare kill it.

Lambert: I can't see a goddamn thing.
Kane: Quit griping!
Lambert: I like griping.

Ripley: Ash... That transmission. Mother's deciphered part of it, it doesn't look like an SOS.
Ash: What is it then?
Ripley: Well, I - it *looks* like a warning. I'm gonna go after them.
Ash: What's the point? I mean - By the time it takes to get there, they'll *know* if it's a warning or not, yes?

Ripley: How do we kill it, Ash? There's gotta be a way of killing it. How... how do we do it?
Ash: You can't.

Ripley: When we throw the switches, how long before the ship blows?
Parker: Ten minutes.
Ripley: No bullshit?
Parker: If we ain't outta here in ten minutes, we won't need no rocket to fly through space.

Ash: You still don't know what you're dealing with do you? Perfect organism. Its structural perfection is matched only by its hostility... I admire its purity, a survivor; unclouded by conscience, remorse or delusions of morality.

Ash: Just one more word. I can't say much for your chances but, [smiling] you have my sympathies.

Ripley: Ash and Captain Dallas are dead. Cargo and ship destroyed. I should reach the frontier in about 6 weeks. With a little luck, the network will pick me up. This is Ripley - last survivor of The Nostromo - signing off.

Ash: There is a clause in the contract which specifically states: Any systematized transmision indicating a possible intelligent origin must be investigated.