Book 31

Conspiracy

by Katherine A Applegate

Published 1 July 1999
The Yeerk that controls Jake's brother Tom is desperate to keep Tom from going with the family to his great-grandfather's funeral, even if he has to kill Jake's father.

Book 32

Separation

by Katherine A Applegate

Published 1 August 1999
Rachel, one of the Earth teenagers dedicated to stopping the parasitic alien race known as the Yeerks, is horrified when she finds herself split into two nearly identical girls.

Book 33

The Illusion

by Katherine A Applegate

Published 1 September 1999
Tobias and the other Animorphs discover that the Yeerks, a parasitic alien race bent on enslaving Earth, plan to test a device that could put an end to the Animorphs' fight against the Yeerks.

Book 34

Prophecy

by Katherine A Applegate

Published 1 October 1999
In an effort to help start a rebellion against the Yeerks on the Hork-Bajir home world, Cassie agrees to become the host for the persona of a dead alien.

Book 35

The Proposal

by Katherine A Applegate

Published 1 November 1999
The fact that his mother has been infested by an alien parasite and his father is planning to remarry puts Marco under such stress that he begins to morph into bizarre creatures that he is afraid will attract the attention of the Yeerks.

Book 36

Mutation

by Katherine A Applegate

Published 1 December 1999
While Jake and the other Animorphs try to stop Visser Three from reaching the Pemalite ship at the bottom of the ocean, they discover a strange underwater civilization.

Book 37

The Weakness

by Katherine A Applegate

Published 1 January 2000
When Tobias discovers Visser Three's newest feeding place, the Animorphs must decide how to proceed.

Book 38

The Arrival

by Katherine A Applegate

Published 1 February 2000
The arrival of Andalites on Earth gives Ax and the Animorphs allies to fight against the Yeerk invasion but also causes new problems.

Book 39

The Hidden

by Katherine A Applegate

Published 1 March 2000
Ax and the Animorphs flee the Yeerks, who seek to find the blue cube that gives the kids the ability to morph.

Book 40

The Other

by Katherine A Applegate

Published 1 April 2000
Ax, who has always believed that he is the only non-infested Andalite on Earth, discovers that there are others trying to survive until help finally comes.

Book 41

Familiar

by Katherine A Applegate

Published 1 May 2000
A boy named Jake wakes up to find himself twenty-five years old and in a world ruled by the aliens known as the Yeerks.

Book 42

Journey

by Katherine A Applegate

Published 1 June 2000
When the Animorphs try to stop the tiny alien Helmacrons from taking the morphing cube, Marco gets in the way and ends up with Helmacrons up his nose and inside his head.