This paper provides a direct test of how fixed export costs and productivity jointly determine firm-level export behavior. Using Chilean data, we construct indices of fixed export costs for each industry-region-year triplet and match them to domestic firms. Our empirical results show that firms facing higher fixed export costs are less likely to export, while those with higher productivity export more. These outcomes are the foundation of the widely-used sorting mechanism in the trade models with firm heterogeneity. A particular and novel finding is that high-productivity nonexporters face greater fixed export costs than low-productivity exporters. We also find that the substitution between fixed export costs and productivity in determining export decisions is weaker for firms with higher productivity. Finally, both larger fixed export costs and greater within-triplet productivity dispersion raise the export volume of the average exporter.