The Collaborative International Dictionary of English v.0.48:

Mistrustful \Mis*trust"ful\, a.
   Having or causing mistrust, suspicions, or forebodings.
   [1913 Webster]

         Their light blown out in some mistrustful wood. --Shak.
   [1913 Webster] -- Mis*trust"ful*ly, adv. --
   Mis*trust"ful*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mistrustfully
    adv 1: with distrust; "she looked at him distrustfully" [syn:
           distrustfully, mistrustfully] [ant: confidingly,
           trustfully, trustingly]