Teachers must nowadays deal with large-scale external testing in most educational systems. Nevertheless, the impact of the uses of external testing by the teachers are still not well known. The aim of this study is to analyse specifically the impact of external testing on teacher’s assessment practices. A qualitative comprehensive analysis was carried out on the experiences of 16 primary teachers in the canton of Fribourg in Switzerland (semi-directive interviews). Results show that teachers may use external tests’ form and contents to adjust their assessment practices. In contrast, they seldom use their results, and the observed adjustments are sometimes made in an instrumental way, to optimize pupils’ achievement to the tests. In the studied context, moderate impact is due to different postures of teachers about external testing and/or to their perceptions of specific characteristics of local devices.