The Collaborative International Dictionary of English v.0.48:

Credulous \Cred"u*lous\ (kr?d"?-l?s; 135), a. [L. credulus, fr.
   credere. See Creed.]
   1. Apt to believe on slight evidence; easily imposed upon;
      unsuspecting. --Landor.
      [1913 Webster]

            Eve, our credulous mother.            --Milton.
      [1913 Webster]

   2. Believed too readily. [Obs.] --Beau. & Fl.
      [1913 Webster]

WordNet (r) 3.0 (2006):

credulous
    adj 1: disposed to believe on little evidence; "the gimmick
           would convince none but the most credulous" [ant:
           incredulous]
    2: showing a lack of judgment or experience; "so credulous he
       believes everything he reads"

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "credulous":
   accepting, apish, asinine, batty, befooled, beguiled, believing,
   besotted, brainless, buffoonish, childlike, cockeyed, confiding,
   crazy, daffy, daft, dazed, dependent, depending, dizzy, doting,
   dumb, dupable, easily taken in, easy of belief, fatuitous, fatuous,
   flaky, fond, fool, foolheaded, foolish, fuddled, futile, gaga,
   goofy, green, guileless, gulled, gullible, idiotic, imbecile,
   inane, inclined to believe, inept, inexperienced, infatuated,
   innocent, insane, kooky, loony, mad, maudlin, moronic, naive,
   nutty, overconfiding, overcredulous, overtrustful, overtrusting,
   reliant, relying, sappy, screwy, senseless, sentimental, silly,
   simple, stupid, superstitious, thoughtless, trustful, trusting,
   trusty, uncritical, undoubting, unquestioning, unskeptical,
   unsophisticated, unsuspecting, unsuspicious, unwary, wacky, wet,
   without suspicion, witless