| Issue | #20 |
| Published | May 1966 |
| Cover Price | 0.12 |
| Pages | 36 |
| Editing | Stan Lee |
| Characters | The X-Men (Professor X, Cyclops, Marvel Girl, Angel, Beast, Iceman); Lucifer; The Blob; Unus |
| Genre | superhero |
| Pencils | Jack Kirby (layouts?); Werner Roth |
| Inks | Dick Ayers |
| Reprinted | in X-Men (Marvel, 1963 series) #71; in Marvel Masterworks (Marvel, 1987 series) #7 |
| Characters | The X-Men (Professor X, Cyclops, Marvel Girl, Angel, Beast, Iceman); V: Lucifer, the Blob, Unus, The Supreme One |
| Genre | superhero |
| Script | Roy Thomas |
| Pencils | Werner Roth (as Jay Gavin) |
| Inks | Dick Ayers |
| Letters | Artie Simek |
| Notes | Story of how Prof. X lost use of legs. Continued in X-Men 21. |
| Reprinted | in X-Men (Marvel, 1963 series) #71; in Marvel Masterworks (Marvel, 1987 series) #7; in Marvel Special (Marvel Italia, 1994 series) #8 |
| Notes | Includes a letter from Frank Brunner |