WordNet (r) 3.0 (2006):

untalkative
    adj 1: temperamentally disinclined to talk [syn: reticent,
           untalkative]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "untalkative":
   brief, brusque, close, close-tongued, closemouthed, concise, curt,
   dumb, economical of words, indisposed to talk, laconic, mum, mute,
   quiet, short, silent, snug, sparing of words, speechless, taciturn,
   terse, tight-lipped, tongue-tied, unloquacious, word-bound,
   wordless