Trichomycetes (Zygomycota) consist of fungi that live within the guts of insects and other arthropods, currently with two orders, Harpellales and Asellariales. Th ese will be the subject of this chapter. Until recently there were four orders within the class, but DNA sequence data indicate that the Amoebidiales and Eccrinales are, in fact, sister clades of protists that fall within the Mesomycetozoea (Benny and O’Donnell, 2000; Cafaro, 2005; White et al., 2006a), a group of mostly unculturable parasites that belong to a region in the tree of life where animals and fungi diverged.