I might be wrong, so wait for Urban to post first. But I think only a few marines are aware of Xenomorphs in this setting.<br/><br/>The first squad, including Tychus being one of them. The reason he is so fucked up, was because he was going to tell people about the aliens but instead got "resocialized" (basically a military ordered lobotomy) to make him forget but he didn't, he just lost a whole lot of his other memories.