The Collaborative International Dictionary of English v.0.48:

Chemotaxis \Chem`o*tax"is\, n. Formerly also Chemiotaxis
\Chem`i*o*tax"is\ [Chemical + Gr. ? arrangement, fr. ? to
   arrange.] (Biol.)
   The sensitiveness exhibited by small free-swimming organisms,
   as bacteria, zoospores of alg[ae], etc., to chemical
   substances held in solution. They may be attracted (positive
   chemotaxis) or repelled (negative chemotaxis). --
   Chem`o*tac"tic, a. -- Chem`o*tac"tic*al*ly, adv.
   [Webster 1913 Suppl.] chemotherapeutic

WordNet (r) 3.0 (2006):

chemotaxis
    n 1: movement by a cell or organism in reaction to a chemical
         stimulus