Alzarius was the homeworld of Adric.

Its coordinates from galactic centre were -10 0 -11 00 by 0 -2, nearly identical to those of Gallifrey. However, it was situated not in the universe containing Gallifrey, but a smaller universe known as E-Space. A starliner from the planet Terradon crash-landed an unspecified number of years before Adric's time.

Alzarius was most notable for its lifeforms' highly unusual biology. This manifested itself most obviously in the extremely rapid adaptive changes made by Alzarian animal life. If placed in an environment it found stressful or even uncomfortable, an Alzarian creature would rapidly develop whatever characteristics it required to make the environment less stressful, such as the ability to breathe in a different medium, or higher muscular density.

Connected to this was the ability to heal injuries at an accelerated rate (although this was less spectacular than the similar Time Lord ability to regenerate). The rate of healing slowed as the individual aged.

Alzarian biology was also unusual in that the boundaries between animal and plant forms of life appeared less rigidly defined: the eggs of the Alzarian spider were discovered developing within riverfruit, apparently as a normal part of the spider's life cycle. (TV: Full Circle)