A humanoid species distantly related to the Ferengi, sharing their resistance to Betazoid telepathy, who are often seen among the visitors on the Promenade of Deep Space 9.
The Dopterians are a short, broad-faced humanoid species with heavy brows and wide nostrils. They are distant cousins of the Ferengi, and like them have minds that Betazoids cannot read.
This is established in the Deep Space Nine episode The Forsaken. A Dopterian thief makes off with Lwaxana Troi's latinum hair brooch at Quark's, and she is unable to pick him out telepathically. Odo deduces the culprit from the kinship between Dopterians and Ferengi and recovers the brooch, which begins Lwaxana's lasting interest in the constable.
Dopterians turn up as background visitors and petty criminals in several later episodes, usually among the crowds on the Promenade or the patrons of Quark's bar.