Given the high importance of the teachers' role in overall school improvement, there has been a strong tradition to investigate various factors affecting teachers' motivation in developed countries. However, like other variables, the factors of intrinsic motivation may offer variance in developed and underdeveloped countries. Thus, this study investigated the importance of work environmental factors and the role of teachers' individual differences in fostering intrinsic motivation by collecting data from 357 Pakistani secondary school teachers. Interpersonal relationships were found to be the highest source of teachers' intrinsic motivation. The structural equation model revealed that school work environment variables: administrative support, working conditions, interpersonal relationships, promotion, colleagues' support, and salary were significant factors in fostering teachers' intrinsic motivation. Teachers' individual differences in gender, academic and professional qualifications, and work experience also showed a significant role in explaining teachers' intrinsic motivation although the extent of effect was smaller than work environmental factors. Female teachers and teachers possessing low qualifications and having work experience of 6-10 years were found more intrinsically motivated. Findings suggest that school administration ought to encourage strategies that foster teachers' needs and expectations, particularly for mid-career teachers and those possessing higher qualifications.