Ash (ID# 111/C2/01X) was the science officer on the commerical towing vessel The Nostromo in the 1979 in film science fiction film Alien. He was played by actor Ian Holm.

As Science Officer, Ash had the final say on everything related to the Company Science Division. This included medical treatment, biological research, and investigation of any possible intelligent life organism encountered. He abruptly replaced the previous science officer, whom Captain Dallas had shipped out with six times before, just as the Nostromo left Thedus for its return journey to Earth. Ash was generally quiet, well mannered, and well spoken. Although Warrant Officer Ripley was suspicious of him, the rest of the crew treated Ash normally. Over Ripley's objections, Dallas entrusted Ash with all science decisions.

Ash violated the Science Division's basic quarantine law (24 hours for decontamination) by opening the main hatch, allowing Kane to enter the ship with the "facehugger" attached to him. Later, he would nonchallantly claim as justification that he was acting on a direct order for Dallas, but when Ripley reminds him that she was the acting command officer at the time, Ash relents, admitting that he took matters into his own hands.

However, the true reason behind Ash's behaviour was Special Order 937—Science Officer Eyes Only, embedded within the ship's computer, "Mother", with whom Ash was constantly "collating". Unknown to the other six Nostromo crewmembers, Ash was really a robot (Hyperdyne Systems 120/A2; a "slightly older model") planted on their vessel to carry out the Company's secret mission detailed in Special Order 937:

SPECIAL ORDER 937: "NOSTROMO REROUTED TO NEW COORDINATES. INVESTIGATE LIFE FORM. GATHER SPECIMEN. PRIORITY ONE. INSURE RETURN OF ORGANISM FOR ANALYSIS. ALL OTHER CONSIDERATIONS SECONDARY. CREW EXPENDABLE."

The "organism" was, of course, the titular Alien. When Ripley assumed command and eventually discovered Special Order 937, Ash attempted to kill her to keep her silent. Indeed, the crew had become expendable. Despite Ash's superior robotic strength, he was eventually outnumbered and overwhelmed by Lambert and Parker, who decapitated him (although his body continued to struggle).

Rather than "killing" him, the decaptiation rather rendered Ash in "negative capacity" for operation. Thus, he was plugged back in and questioned by the three remaining crewmembers (the remainder had been either killed or captured by the alien). Ash spoke of his admiration for the species: "A perfect organism... structural perfection.... I admire its purity. A survivor unclouded by conscience, remorse, or delusions of morality." He parted with the words: "I can't lie to you about your chances, but (smirkingly) you have my sympathies."

Ash was rendered permently inoperative when Parker incinerated him with a flamethrower unit.

Though Ash had given valid information and vital assitance to the crew, he had also been protecting the alien from the beginning. In fact, every action taken by Ash was designed to get the alien on board the Nostromo for the return trip to Earth. In a way, Ash was "protecting them from each other", so as to minimize any damage brought on by inter-species contact.

Like the Nostromo itself, Ash and all other crewmembers' official whereabouts remained unknown for the next 57 years. He was missing and presumed destroyed from 2122 until 2179, when an off-course Narcissus escape ship returned Ripley and Jones to civilization (and solved the riddle of the missing Nostromo, which had mysteriously vanished without a trace).