Samples from directionally solidified Al- 7 wt. % Si have been analyzed for primary dendrite arm spacing (A). The alloy was directionally solidified (DS) aboard the International Space Station (ISS) to determine the effect of mitigating convection on λ. In some experiments, λ was measured in microstructures that developed during the transition from one speed to another. To represent DS in the presence of no convection, the Hunt-Lu model was used to represent diffusion-controlled growth. By sectioning cross-sections throughout the entire length of a solidified sample, λ. was measured and calculated using the model. During steady state, there was reasonable agreement between the measured and calculated λ's in the space-grown samples. Where there was evidence of Marangoni convection, however, resulted in primary dendrite arm spacings that were larger than they would have been in the absence of convection.