Irregular eutectic growth in an Al-Si eutectic alloy is investigated using directional solidification. The high-rate transition from flake to fiber morphology is characterized in terms of the three dimensional structure. The flake to fiber transition is observed to occur over the velocity range from 75-850 [mu]m/s. Average eutectic spacing measurements were made using lineal and areal methods, with both exhibiting the Jackson and Hunt growth relationship of [lambda]2v=Constant. Both methods accurately described average eutectic growth, but did not clearly indicate a morphological transition. The two regimes of growth behavior are observed, however, using a characteristic length scale defined by the spatial variation in local volume fraction. The flake to fiber transition is also characterized in terms of particle aspect ratio, interface undercooling, and Si phase fraction in the eutectic. Branching methods of the Si particles are identified throughout the range of velocities studied.