A spacefaring species who breathe a corrosive mixture of nitrogen, benzene and hydrogen fluoride, and whose ambassador's expected visit caused headaches for the staff of Deep Space 9 in 2371.
The Yalosians are a species mentioned, but never seen, in the Deep Space Nine episode Improbable Cause. Their biology is far removed from that of most of the station's visitors.
At the start of the episode Kira and O'Brien are preparing guest quarters for a Yalosian ambassador. The delegation breathes a mixture of roughly sixty per cent nitrogen, ten per cent benzene and thirty per cent hydrogen fluoride, and a trial of that atmosphere in empty quarters dissolved the carpet. Yalosians also cannot see the red-orange part of the spectrum, which complicates the decoration.
The conversation is cut short when Garak's tailor shop explodes on the Promenade, opening the investigation that leads Odo and Garak towards the Tal Shiar and the Obsidian Order.