The Yalosians are a sapient extraterrestrial species which breathes an atmosphere composed of 60% nitrogen, 10% benzene and 30% hydrogen fluoride. In 2371, a Yalosian ambassador payed a visit to the Deep Space 9 station an had special quarters prepared for him. However, Major Kira found out that the Yalosian atmosphere was so corrosive she had to remove the carpet because it was being damaged. Dr. Bashir also advised her not to include any red or orange decoration, because Yalosians are not able to see that part of the light spectrum.
No known Yalosian has ever appeared in canon. The visit from the Yalosian ambassador was discussed briefly in the Deep Space Nine episode "Improbable Cause"