This paper investigated the tearn-teaching at three primary schools in Korea. The data consisted of classroom audio-recordings, in-class observations and post-observation interviews. This study first examined the degree to which Teaching English through English (TEE) was practiced by Native English Teachers (NETs) and Korean English Teachers (KETs) and the extent to which each case fit five conditions for optimal language learning environments: interaction with an authentic audience; opportunity for the negotiation of meaning; exposure to creative and varied L2; production of creative and varied L2; and engagement in authentic tasks (Egbert & Hanson-Smith, 1999). The main findings were that in five different observation sites, as expected, L I was rarely used in NET classes, but was used in 28% of utterances in KET classes, often to perform functions for which L2 English could have easily been used. In addition, it was found that very few instances of NET or KET classes met any of the five conditions for optimal language learning environments. This research proposes that L2 English be maximized in class, and that teachers pay more attention to creating a learning environment that satisfies the conditions for optimal language learning.