The only member of its genus, Allosmerus elongatus, the whitebait smelt, is an uncommon marine species of smelt, about which little is known. Originally described as both Osmerus attenuatus and O. elongatus, these two species were determined to be conspecific in 1946. The fish can grow from 7–9 inches in length, has large eyes, a greenish-gray color on its back, and a silver band along its sides. Unlike most other smelt species which generally have no enlarged teeth in the roof of their mouth, the whitebait has single large tooth in the center of its vomer, which is sometimes flanked by a smaller tooth on either side. The adult males of the species have a longer anal fin.

The only member of its genus, Allosmerus elongatus, the whitebait smelt, is an uncommon marine species of smelt, about which little is known. Originally described as both Osmerus attenuatus and O. elongatus, these two species were determined to be conspecific in 1946. The fish can grow from 7–9 inches in length, has large eyes, a greenish-gray color on its back, and a silver band along its sides. Unlike most other smelt species which generally have no enlarged teeth in the roof of their mouth, the whitebait has single large tooth in the center of its vomer, which is sometimes flanked by a smaller tooth on either side. The adult males of the species have a longer anal fin.