Unstructured learner interaction (ULI) refers to the use of a foreign language by learners who share an L1 with their peers outside the classroom in the L 1 environment. The introduction to this thesis recounts the researcher's experiences with learners who have developed their proficiency through ULI, and it raises the question of why ULI is not common in Japan, even among highly motivated learners. The literature review is presented in two parts: Chapter 2 examines studies of interaction from a historical perspective, and Chapter 3 investigates other fields of relevance. The main body of the thesis is made up of two studies. In Study 1, approximately 50 Japanese university students were surveyed to find out more about their attitudes to ULI. Their progress was then charted over four weeks as they attempted to use English outside the classroom. Data were collected through diaries, interviews, and the researcher's observations. The main findings of the study were that many students were able to use English outside the classroom. Reported benefits included gains in fluency, increased motivation, and reinforcement of previous learning. The findings of Study 1 were collated to create guidelines for the promotion of ULI within educational institutions. Study 2 compared unstructured and structured interactions between 16 pairs of learners. These were a different group from the participants in Study 1, but they were taken from a similar population. The findings were that the learners took significantly longer turns in ULI and produced more complex language, but that there was no difference in speech rate or amount spoken. The conclusion was that the benefits of communicative activities conducted in the language classroom should also accrue from ULI, and that ULI may offer a valuable source of practice for learners who do not have regular contact with fluent speakers of English. The thesis concludes with a call for increased awareness of the potential benefits of ULI among both learners and teachers of foreign languages.