This longitudinal study investigated teachers' use of the first language (L1) in two French foreign language (FL) intermediate level classes at two Australian universities. A native French-speaking teacher (NS) and a non-native French-speaking teacher (NNS) were observed and audio-recorded approximately every two weeks over a 12- week semester. The study investigated the quantity of L1 used, the purposes it served, and whether these changed over time. It also investigated possible factors that explain teachers' L1 use, including teachers' attitudes towards the L1 use. The study found consistently more L1 use by the NNS than the NS, but that over time the use of L1 by the NS teacher increased. The L1 served a variety of purposes, most notably explanations of vocabulary and of tasks. However, towards the end of the semester, there was a marked increase in the use of L1 by both teachers for the purpose of general administration. The interview data show that use of L1 was related to the teachers' pedagogical beliefs and goals and these together with context-specific factors determined L1 use. The findings are discussed in terms of how to establish guidelines concerning appropriate levels of L1 use in FL classes. (Contains 6 tables.)