This paper examines the effect of trade liberalization on plant scale, production-run length and product diversification. We first develop a model of trade in differentiated products with multi-product plants. We then present empirical evidence using a large panel of Canadian manufacturing plants and their experience with the 1989 Canada-U.S. Free Trade Agreement (FTA). The model predicts that the bilateral tariff reduction reduces the product diversification of exporting plants, increases the production-run length and has an ambiguous effect on the size of those plants. It also reduces the product diversification and size of non-exporting plants, and has no effect on the production-run length of those plants. The empirical evidence on non-exporting plants provides broad support for the model. The evidence on exporting plants shows that exporters reduce product diversification, and increase production-run length and plant size, but those changes do not appear to be related to tariff cuts. Once in the export markets, plants respond to forces other than tariff cuts. Further tariff cuts have less effect on those plants.