The article tells about methodological problems of teaching second foreign language at secondary education institutions. The paper analyzes phenomena of interference which arise under conditions of trilinguism. Comparing phonetic, lexical and grammatical systems of the Russian, English and French languages, the author specifies the sources of interference in teaching French as a second foreign language with the English as the first one. The article gives teachers methodological recommendations with the aim of predicting and avoiding negative impact that linguistic skills in the native and English languages have on forming automatic features in learning French.