The Collaborative International Dictionary of English v.0.48:

Incredulous \In*cred"u*lous\ (?; 135), a. [L. incredulus. See
   In- not, and Credulous.]
   [1913 Webster]
   1. Not credulous; indisposed to admit or accept that which is
      related as true, skeptical; unbelieving. --Bacon.
      [1913 Webster]

            A fantastical incredulous fool.       --Bp. Wilkins.
      [1913 Webster]

   2. Indicating, or caused by, disbelief or incredulity. "An
      incredulous smile." --Longfellow.
      [1913 Webster]

   3. Incredible; not easy to be believed. [R.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

incredulous
    adj 1: not disposed or willing to believe; unbelieving [ant:
           credulous]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "incredulous":
   Humean, Pyrrhonic, agnostic, cautious, creedless, disbelieving,
   disinclined to believe, disposed to doubt, distrustful, doubtful,
   doubting, dubious, faithless, guarded, hard of belief, heretical,
   hesitant, impervious to persuasion, inconvincible, irreligious,
   leery, minimifidian, mistrustful, nonbelieving, nullifidian,
   questioning, quizzical, repudiative, sceptical, shy of belief,
   skeptic, skeptical, suspecting, suspicious, unbelieving, uncertain,
   unconfident, unconverted, unconvinced, unconvincible, uncredulous,
   unpersuadable, unpersuasible, unsatisfied, unwilling to accept,
   wary, without faith