You can wild shape into vermin form.
Prerequisites: Knowledge (nature) 24 ranks, wild shape 6/day.
Benefit: The character can use his or her normal wild shape ability to take the form of a vermin. The size limitation is the same as the character’s limitation on animal size.
Source: Standard Reference Document under Open Game License v1.0a, which updated this feat originally published in the Epic Level Handbook, page 68.