Directional solidification studies using a constant temperature gradient of 125 +/- 5-degrees-C cm-1 in the liquid form the main part of this study. They are used to define the effect of Sr in the concentration range 0-0.1 wt% on the structure of Al-Si alloys containing 11-20 wt% Si and solidified in the velocity range 1-1500-mu-m s-1. The high temperature gradient suppresses the nucleation of Si so the studies were complemented with observations of primary Si phase solidification under a low temperature gradient. Very low Sr concentrations induce structural modifications of the Si phase that depend upon the Si concentration and the growth velocity for a fixed temperature gradient. The action of Sr is attributed to the poisoning of growth sites rather than the prevention of nucleation. Twin-plane re-entrant (TPRE) growth sites in primary Si are readily poisoned, whereas the effect on layer growth sites is more gradual producing holes, branching and dendritic structures and dispersed equiaxed structures as the Sr concentration increases. Growth twins were observed in these structures but did not activate the TPRE growth mechanism. Sr broadens the coupled zone and modifies the anomalous Si flake eutectic to a  fibrous structure. Multiple growth twins were observed in some of the fibres and they play an important role in their growth. Some of the fibres develop from angular Si by splitting or branching in the plane of the  side plates. These fibres do not show twinning. Transitional eutectic structures were observed consisting of modified complex regular phases and modified angular Si. These structures confer a  texture and their occurrence depends on the Si and Sr concentration, growth velocity and temperature gradient.