This paper studies the differences in the uses and effects of U.S. antidumping law on imports and domestic output across the major regions exporting to the United States. Building on previous work (Staiger and Wolak, 1994), we extend our attempt to characterize the implications of the use of antidumping law for the behavior of U.S. imports and domestic output, and to distinguish between "outcome filers" (firms for which the prospect of an antidumping duty is an important ingredient in the decision to file) and "process filers" (firms for which filing is driven largely by a desire to secure the trade-restricting effects of the investigation process itself). In our earlier work we allowed for the coexistence of outcomeand process-filing industries, and found evidence consistent with the presence of process filers in some industries. However, we restricted the filing strategy of firms in a given domestic industry to be the same across all imports in that industry regardless of their country of origin. In this paper we abstract from cross-industry heterogeneity in antidumping filing strategies and explore instead the heterogeneity of filing strategies against different importsource countries, allowing for the possibility that domestic firms may pursue independent filing strategies with respect to imports from different countries. We argue that the most likely target countries for process filers are those whose export production is primarily destined for the U.S. market and accounts for a relatively large and stable U.S. market share. These characteristics point to Canada and Mexico as countries against which process filing by U.S. firms is likely to occur. Analyzing the filing behavior against Canada and Mexico as well as four other regions, we find evidence in the filing behavior and in the nature of the trade impacts which accompany filing to suggest that Mexico and Canada are indeed the most likely targets of antidumping petitions filed by process filers in the United States.