The Collaborative International Dictionary of English v.0.48:

Taxonomist \Tax*on"o*mist\ (t[a^]ks*[o^]n"[-o]*m[i^]st), n.
   One skilled in taxonomy.
   [1913 Webster]

WordNet (r) 3.0 (2006):

taxonomist
    n 1: a biologist who specializes in the classification of
         organisms into groups on the basis of their structure and
         origin and behavior [syn: taxonomist, taxonomer,
         systematist]