The Longnose Butterflyfish has a compressed yellow body with a tiny mouth at the end of a very long snout. The upper part of head is black or grey and the snout is silvery-white. A black spot present on the rear of the anal fin just below the caudal peduncle and a black bar runs from the beginning of the dorsal fin through the rear part of the gill cover to the pectoral-fin base. Chest usually with rows of black flecks or spots.
A similar species, the Yellow Longnose Butterflyfish (F. flavissimus) this fish has a shorter snout, and a slightly larger mouth, than that of F. longirostris. The Yellow Longnose Butterflyfish also has a higher dorsal spine count (12-13) compared to the Big Longnose (10-11). They are also known for getting rid of Aiptasia or Glass Anemones. They are peaceful, and are suitable to be kept with most fish species, but should not be housed with members of its own genus, or overly territorial species.