The Collaborative International Dictionary of English v.0.48:

naturalist \nat"u*ral*ist\, n. [Cf. F. naturaliste.]
   1. One versed in natural science; a student of natural
      history, esp. of the natural history of plants or animals;
      a botanist or zoologist.
      [1913 Webster +PJC]

   2. One who holds or maintains the doctrine of naturalism in
      religion. --H. Bushnell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

naturalist
    n 1: an advocate of the doctrine that the world can be
         understood in scientific terms
    2: a biologist knowledgeable about natural history (especially
       botany and zoology) [syn: naturalist, natural scientist]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "naturalist":
   Marxist, anatomist, bacteriologist, biochemist, biologist,
   biometrist, biophysicist, botanist, commonsense philosopher,
   cytologist, dialectical materialist, ecologist,
   epistemological realist, geneticist, materialist, natural realist,
   natural scientist, physicist, physiologist, realist, zoologist