TYPE SPECIES:
• A. ajax Marsh, 1877 "Ajax (hero of the Trojan War) deceptive lizard"
  OTHER SPECIES:
• A. excelsus (Marsh, 1879) (originally Brontosaurus)
• A. louisae Holland, 1915
• A. montanus (Marsh, 1877) [nomen dubium] (originally "Titanosaurus") "montane deceptive lizard"
  
  MISASSIGNED SPECIES:
• A. alenquerensis de Lapparent & Zbyszewski, 1957 = Lourinhasaurus alenquerensis
• A. amplus Marsh, 1881 == A. excelsus
• A. grandis (Marsh, 1877) (originally Camarasaurus) "grand deceptive lizard" = Camarasaurus grandis
• A. laticollis Marsh, 1879 == A. ajax
• A. minimus Mook, 1917 [nomen dubium] "least deceptive lizard" = Haplocanthosaurus minimus
• A. yahnahpin Filla, James & Redman, 1994 = Eobrontosaurus yahnahpin

Animalia
Vertebrata
Tetrapoda
Sauropsida
Archosauromorpha
Ornithodira
Dinosauria
Sauropodomorpha
Sauropoda
Diplodocimorpha
Diplodocidae

This genus is more popularly known as Brontosaurus, but Brontosaurus has long been considered a junior synonym of Apatosaurus. There is a minority opinion that A. excelsus does represent a valid genus (named Brontosaurus) separate from A. ajax.

Apatosaurus was shorter but bulkier than its relatives Barosaurus and Diplodocus.