A high purity Fe-C alloy of eutectoid composition has been transformed from homogenized austenite in two ways: isothermally and at forced constant-velocity. The latter transformation mode, forced over five decades of velocity, produces structures with characteristic dimensions ranging over nearly two orders of magnitude. There is a consistent alignment of the transformation product in the heat-flow direction at all velocities, but the classical pearlite morphology undergoes a microstructural transition at the higher velocities. A spacing-velocity relationship for regular lamellar pearlite was measured as V0.41±o.02λν = constant.