In this study the mechanical properties of bacterial biofilms of mucoid Pseudomonas aeruginosa strains have been investigated. A home-built instrument is used for the rheological investigation of the agar grown biofilms. The main experimental routine allows the determination of biofilm thickness, an apparent modulus of elasticity and the yield stress in uniaxial compression. Also stress relaxation and experiments with oscillating deformation are performed. Depending on concentration of multivalent cations the biofilm behave as viscoelastic fluids or viscoelastic solids, respectively. Interactions between cations (e.g. Ca2+) and extracellular polymeric substances (EPS) influence the mechanical properties of the biofilms. Further influences investigated in this study are the effect of aqueous solutions and the inclusion of particles (glass beads) in the biofilm.