Potential influences on the student teaching experience relate to the characteristics of the placement site, the supervision of the student teacher, the requirements of the program, and the abilities of the student teacher. This study of the ecological factors that influence a student teacher's development examined the evolution of student teaching relative to increased responsibilities and the factors that influence this development. Data was gathered on six student teachers' activities through two teaching assignments, their performance and their perceptions of their experiences. Information was sought on how the subjects made the transition from observing to teaching, how this differed in the two teaching assignments, how they perceived their experiences, and whether their performance in student teaching affected the transition from observing to teaching. Information was gathered by means of daily logs on activities, evaluations of student performances, and weekly logs describing their perceptions. A discussion of the findings points out the importance of careful structuring of student teacher activities to facilitate the transition from observing to teaching. (JD)