  We have derived exact Langevin equations for a model of quasispecies
dynamics. The inherent multiplicative reaction noise is complex and its
statistical properties are specified completely. The numerical simulation of
the complex Langevin equations is carried out using the Cholesky decomposition
for the noise covariance matrix. This internal noise, which is due to
diffusion-limited reactions, produces unavoidable spatio-temporal density
fluctuations about the mean field value. In two dimensions, this noise strictly
vanishes only in the perfectly mixed limit, a situation difficult to attain in
practice.
