The transient elongational behavior of a highly‐aligned 60% fiber volume fraction long‐discontinuous‐fiber filled poly‐ether‐ketone‐ketone (PEKK) melt system was investigated with a computer controlled extensional rheometer at 370 °C. Long discontinuous graphite fibers, 7 μm in diameter, and PEKK thermoplastic constituted the melt system. Experiments conducted under conditions to approximate controlled strain rate and controlled stress produced τ̄+E(t,ε̇̄) and ε̇̄−(t,τE) similar to a shear dominated flow of a non‐linear viscoelastic fluid. Increasing strain rate resulted in an increasing peak stress and in an increasing ratio between peak stress and plateau stress. Increasing controlled stress level resulted in decreasing elongational viscosity and increasing post‐elastic‐step strain rate. The ratio of apparent ηE/ηPEKKwas 3.67×108to 3.94×1010for average strain rates from 6.81×10−7to 1.49×10−3s−1. A variable taper shear cell analysis provided the range of expected ηE/η, which agreed well at high strain rates with the experimental data.