The behavior of sepiolite (magnesium silicate with a microfibrillar morphology) as a filler in ethylene-propylene-diene compounds was studied. A new coupling agent, meta-isopropenyl dimethyl benzyl isocyanate, was used. The effect of the filler modified by meta-isopropenyl dimethyl benzyl isocyanate on the physical properties was improved with respect to other fillers and commercial silane coupling agents under the same conditions. In addition to a smoother surface, the flow behavior of compounds filled with sepiolite, on account of its fibrous morphology, was better than that obtained with other fillers. (C) 2003 Wiley Periodicals, Inc. J Appl Polyrn Sci 91: 1489-1493, 2004.