The extensional effective viscosity, ηe, of a suspension of solid cylindrical fibers in a Newtonian liquid is measured from the gravitational stretching of a quasi-steady jet. We vary the lengthto-diameter aspect ratio of the fibers, a, from ∼ 10 to ∼ 10^2 , and the particle volume fraction, φ, in the range aφ ∼ 0.1 − 2. For low values of φ, the extensional viscosity is found to agree with Batchelor's model for the dilute regime, which assumes non-interacting fibers aligned with the stretching direction, but for higher concentrations, ηe is found to increase much more strongly with increasing φ than predicted by available models assuming purely hydrodynamics interactions between the fibers. Additional experiments are performed on the breakup of an unstable capillary bridge for a = 11. Although the variability in the bridge shape and the deviation from a viscous Newtonian dynamics in the late stage of the breakup increase significantly with increasing φ, it is found that the mean total duration of the breakup is in good agreement with a Newtonian effective dynamics limited by the extensional viscosity, ηe.