Physically, the race were humanoid, although they had a pale, almost metallic cast to their skin. They used holographic technology and created sentient holograms to act as cleaners and maintenance personnel aboard their starships. Unfortunately, while these programs, referred to as isomorphs, were given individuality to the point where each was given their own name, they were not well-treated by their creators, who viewed them as little more than tools.
In 2374 the USS Voyager received a distress call from a Serosian hologram called Dejaren. Although he claimed the crew of his ship had been killed by a virus, it later became apparent that his program had destabilized and he had become obsessed with cleanliness to the point of psychosis. As a result he had murdered the crew, disgusted at their organic nature. (VOY: "Revulsion")