| Issue | #21 |
| Published | June 1966 |
| Cover Price | 0.12 |
| Pages | 36 |
| Editing | Stan Lee |
| Notes | Disputed penciler and inker credits--Jack Kirby (layouts) and Werner Roth pencils with Dick Ayers inks per the original indexer; Werner Roth pencils only with Dick Ayers inks per Carl Gafford; Werner Roth pencils and inks per Steve Whitaker and Nick Caputo. |
| Characters | The X-Men (Professor X, Cyclops, Marvel Girl, Angel, Beast, Iceman); Lucifer |
| Genre | superhero |
| Pencils | Werner Roth ? |
| Inks | Dick Ayers ?; Werner Roth ? |
| Notes | Disputed penciler and inker credits--Jack Kirby (layouts) and Werner Roth pencils with Dick Ayers inks per the original indexer; Werner Roth pencils only with Dick Ayers inks per Carl Gafford and Nick Caputo; Werner Roth pencils and inks per Steve Whitaker. |
| Reprinted | in X-Men (Marvel, 1963 series) #72; in Marvel Masterworks (Marvel, 1987 series) #7 |
| Characters | The X-Men (Professor X, Cyclops, Marvel Girl, Angel, Beast, Iceman); V: Lucifer, The Supreme One |
| Genre | superhero |
| Script | Roy Thomas |
| Pencils | Werner Roth (as Jay Gavin) |
| Inks | Dick Ayers |
| Letters | Artie Simek |
| Reprinted | in X-Men (Marvel, 1963 series) #72; in Marvel Masterworks (Marvel, 1987 series) #7; in Marvel Special (Marvel Italia, 1994 series) #8 |
| Letters | typeset |
| Notes | Letters from John Keppler, Gordon Flagg, Syd Terry, Bill Tipton, Guy Michael, Fred Waiss, Charles McClain, Lewis Torro, Laura Lindh and Ken Chew with accompanying illustrations. |