WordNet (r) 3.0 (2006):

untrusting
    adj 1: openly distrustful and unwilling to confide [syn:
           leery, mistrustful, suspicious, untrusting,
           wary]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "untrusting":
   Pyrrhonic, agnostic, distrustful, doubtful, doubting, dubious,
   from Missouri, in doubt, leery, mistrustful, mistrusting,
   questioning, scrupulous, shy, skeptical, suspecting, suspicious,
   uncertain, wary