Polymer processes which employ elongational flows have been successfully used for shaping ceramics by using crowded (50–60 vol %) suspensions of ceramic particles, but the full realization of their potential rests on an understanding of the extensional flow behavior of filled polymer systems. A uniaxial rheometer was constructed to evaluate the extensional flow properties of crowded suspensions based on polyisobutylene. The shear viscosity was evaluated for comparison using a Weissenberg rheometer and correlated with published literature values for a similar molecular weight polyisobutylene. Trouton’s Law was verified for the unfilled polymer. The shear and elongational viscosities of filled polyisobutylene, containing up to 42 vol % and 45 vol % ceramic, respectively, were measured. The dependence of shear and elongational relative viscosities on powder volume fraction were compared. In uniaxial flow, Trouton’s Law was obeyed for crowded suspensions containing up to 42 vol % ceramic powder.