The flotation separation of ultrafine (-5 mu m) sulphide mineral particles is inherently problematic, since even strongly hydrophobic fines have relatively low flotation rate constants. Selective aggregation of fines provides a route for enhanced flotation selectivity, but requires detailed knowledge and ultimately control of the electrostatic and hydrophobic inter-particle forces acting in concentrated sulphide mineral slurries. Rheological investigations offer insight into these inter-particle interactions. Rheological studies of ultrafine sphalerite particle slurries at pulp densities akin To those experienced in flotation practice were undertaken; the influence of pH, electrolyte concentration, activating copper (II) ions and ethyl xanthate ions on the level of pulp aggregation, as defined by the yield values and viscosities, are reported. The rheological behaviour of oxidised, and effectively hydrophillic, sphalerite particles (in the pH range 4 to 10) are controlled by electrostatic repulsive forces and yield values scale inversely with electrophoretic mobilities. Copper (II) activation, ethyl xanthate treatment and addition of slime particles dramatically affect pulp theology and, depending on the particular concentrations and conditions employed, cause either aggregation or dispersion of the pulp. We show that an understanding of the theological behaviour of sulphide mineral pulps enhance our knowledge of particle-particle (and bubble-particle interactions) and may enable fine particle flotation performance to be optimised.