We describe a longitudinal study of a secondary education biology teacher at two moments in her career (1993-2002), determining the changes in her conceptions of the nature of science and its teaching and learning, and the factors that favored or hindered such changes. The changes were analyzed using cognitive maps, constructed on the basis of the Inventory of Teachers' Pedagogical & Scientific Beliefs (INPECIP) questionnaire, designed and validated by R. Porlan, A. Rivero, and R. Martin del Pozo (1997), and a semistructured interview. The results showed the process of change in conceptions to be complex and gradual, with different conceptions being out of phase with each other. During her first 4 years of teaching, until 1993, her conceptions of science teaching and learning began to evolve from a teacher and content-centered model to one that was more student centered. The catalyst of the change in her initial conceptions of teaching and learning was her becoming aware of the students' alternative ideas. Her empiricist conception of the nature of science, however, remained practically unaltered during these first 4 years. This conception began to shift slowly toward less dogmatic and more up-to-date positions as a consequence of her changing view of teaching-learning, and by 2002, there was again coherence between her scientific thinking and her ideas on science teaching and learning.