This paper shows that the share of exports in the total sales of a firm has a positive and substantial impact on the volatility of its sales. Decomposing the volatility of sales of exporters between their domestic and export markets, I show using an identification strategy based on a firm-specific geographical instrument that firms with a larger export share have more volatile domestic sales and less volatile exports. These empirical patterns can be explained using a model in which firms face market-specific shocks and short-run convex costs of production. In such a framework, firms react to a shock in one market by adjusting their sales in the other market. I point to strong evidence that output variations on the domestic and export market are negatively correlated at the firm level. This result casts doubts on the standard hypothesis that firms face constant marginal costs and maximize profits on their different markets independently of each other. Furthermore, it points to the caveat that sales volatility on a particular market only gives limited information about the size of shocks on that market.