Calixtus I
- Biographical profile of the third-century pope, in the 1911 Encyclopædia Britannica.
- http://encyclopedia.jrank.org/CAL_CAR/CALIXTUS_I.html

Callistus (Callixtus) I, Pope
- Biographical portrait.
- http://www.saintpatrickdc.org/ss/1014.htm#call

Catholic Online: St. Callistus I
- Hagiography, by Terry Matz.
- http://www.catholic.org/saints/saint.php?saint_id=31

Callixtus I
- Short biography, by Karen Rae Keck. It was Callixtus who excommunicated Sabellius.
- http://www2.evansville.edu/ecoleweb/glossary/callixtusi.html

Vietnamese Eucharistic Youth Society: St. Callistus I
- The story of this pope, suited to children.
- http://www.tntt.org/vni/tlieu/saints/St1014.htm

St Calixtus, or Callistus, Pope, Martyr
- From Butler's Lives of the Saints.
- http://www.ewtn.com/library/MARY/CALLIXTU.htm

St. Calixtus I
- This pope is known mostly through the writings of his enemies.
- http://www.cfpeople.org/Books/Pope/POPEp16.htm

Callistus I
- Illustrated profile, with links.
- http://www.catholic-forum.com/saints/saintc36.htm

Pope Callistus I
- Martyr, d. about 223. Also known as Callixtus or Calixtus.
- http://www.newadvent.org/cathen/03183d.htm