Muon-spin-rotation experiments have been carried out in single crystals of the heavy-fermion compound CeRu2Si2. The muon-stopping site has been determined from an analysis of the muon Knight shift. The zero-field relaxation rate of the muon polarization exhibits a small but significant increase below approximately 2 K which can be suppressed in longitudinal-field experiments. This result provides definite evidence for the occurrence of static electronic magnetism involving ultrasmall moments of the order of 10(-3)mu(B).