The viscoelastic properties of gelling Laponite clay suspensions were studied by measuring the time evolution of the frequency dependent shear modulus. At a well defined gel time the loss and storage modulus have a power law frequency dependence with the same exponent Delta = 0.55. The exponent is compared with theoretical predictions for fully flexible and locally rigid gels. After the gel point the shear modulus continues to evolve and a stable state was not reached in the experiments. The loss modulus of the gel increases with decreasing frequency at low frequencies, indicating the presence of slow relaxation processes. The rate of gelation increases strongly with increasing ionic strength. (C) 2000 The Society of Rheology. [S0148-6055(00)00703-3].