The viscosities of concentrated suspensions of glass fibers in high viscosity silicone oils were measured with a14″capillary viscometer. Suspensions were mixed by various techniques, but only data for extruder mixed suspensions are presented. All pertinent suspension parameters were monitored, including wall effects. Yield stresses, squeeze‐through, and capillary entrance exclusion were observed. Both the quantitative and qualitative aspects of fiber suspension rheology depended strongly on fiber length, which is consistent with the variety of behavior reported in the literature. Controlling fiber length independent of changes in other suspension parameters is difficult.