The chigger is of importance to humans and animals as an irritating pest and a crucial vector of disease. We observed stylostome formation by larvae of Leptotrombidium pallidum in parasitized striped-field mouse (Apodemus agarius) pinna skin. The stylostome formations were also examined in detail by electron microscopy. The proximal and middle parts of the stylostome were formed as a rod-like structure containing a canal. The stylostome was coated with thick degenerated epidermal cells, and seemed to be formed a hyperplastic epidermis. The stylostome formation may result from an interaction between larval secretions and the host tissue. According to histological and morphological characteristics, the stylostome exhibited considerable inflammation in the epidermis and similar reactivity.