The lattice parameters have been determined as a function of temperature for single crystals of the ferroelectric PbZr0.9Ti0.1O3. By associating the difference in rhombohedral angle with the square of the order parameter, Ps, the critical index beta has been determined for the ferroelectric to paraelectric phase transition. This transition appears to be of second order with beta approximately=0.33 for (T0-T)/T0<or=0.06. This difference from the mean-field value ( beta =0.5) suggests the presence of critical fluctuations in the order parameter. Outside the critical region the order parameter deviates even further from mean-field behaviour. This is explained in terms of further fluctuations associated with a transition at a lower temperature.