| 1a. | Photophores present on body or under head | 5 |
| 1b. | Photophores absent | 2 |
| 2a. | Dorsal fin base many times longer than head length | Family Alepisauridae Lancetfishes |
| 2b. | Dorsal fin base shorter than or about equal to head length | 3 |
| 3a. | Pelvic axillary process (fleshy appendage at base of pelvic fin) present | Family Salmonidae Salmons and Trouts |
| 3b. | Pelvic axillary process absent | 4 |
| 4a. | Jaws extending to under or slightly behind eye; jaw teeth small | Family Osmeridae Smelts |
| 4b. | Jaws extending well behind eye; jaw teeth large, caninelike | Family Synodontidae Lizardfishes |
| 5a. | Photophores in a duct between lower jaw and pelvic fins; scales tubelike structures along lateral line, otherwise scaleless | Family Paralepididae Barracudinas |
| 5b. | Photophores on body, below lateral line | Family Myctophidae Lanternfishes |