Mechanical and rheological properties of filled polymers are very sensitive to the filler dispersion. Classical methods for evaluating dispersion are based on optical or scanning electron microscopy observations on extruded specimens. They cannot be used in-line and are generally time consuming. We present here an in-line characterisation technique to evaluate the dispersion of calcium carbonate, CaCO3, in polypropylene. The technique is based on the monitoring of velocity and attenuation of ultrasounds in the flowing material. These properties are measured in a specially instrumented die placed at the end of a single screw extruder. Measurements were made for filler concentrations up to 60 wt.%. In-line data are compared to dispersion indexes measured by an optical method on 25 micron thick films.