This paper is based on ongoing research on a recent low-dose, late partial Englishimmersion (IM) program in Germany. The evaluation compares English language outcomes ofIM groups, groups from non-IM schools, and non-IM groups from the same school as the IMgroups, at various points of their development. This paper focuses on whether Englishvocabulary learning occurs incidentally while students are learning history or geography, or both,taught in English and whether there is evidence to suggest that the learning abilities activated inthe IM classroom are the same as those found in traditional foreign language teaching and innaturalistic (untutored) L2 acquisition. The data derive from a communicative group test. It isshown that some of the lexical items cannot have come from the textbook or from other kinds ofteaching materials used during regular foreign language instruction in the program. This leavesthe teacher's oral use of English as the most likely source. Several implications for L2acquisition theory and teaching practice are discussed.