At Universidad Pedagogica Nacional de Mexico (UPNM) like in other educative institutions in the world, investigation programs are developed which has as main concern' professor's formation. Investigation on biology professors' explanation models in described in this document. At their explanations, professors use many previous ideas. These ideas are strongly retained and usually they stay unchanged at indoctrination (Osborne and Freyberg, 1985) and they behave as genuine obstacles that difficult the concept's development (Driver, 1978). Regularities and consistencies regarding this ideas, between other very similar ideas, witch are contradictory and dissimilar to scientific concept have been observed and recorded (Colin, 1991). Professors need to know the phenomena, concepts and methods, principles and theories that constitute the science they teach (Hewson and Hewson, 1987). As observed, the importance of investigating previous ideas which are present in professors explanation models, is given by the repercussions they have in student's knowledge formation, nevertheless this is an aspect yet few studied. The investigation was made with 114 biology's professors, from Mexico City's high schools. The group shows a wide heterogeneity on professor's academic profile. Of the population studied, only 37.71% are biologists, whilst 39.47% are titulated. Methodology This work has been based on Treagust (1988) and Yip's (1998) methodological proposals. The following activities were developed: 1. Enouncement identification with prepositional knowledge. 2. Conceptual map tracing. 3. Relating enunciations to conceptual maps. 4. Valuation of contents. 5. Testing literature related. 6. Un-prepared interview to biology's professors. 7. Writing test enouncements. 8. Pilotaje, reliability test and questionnaire adjustment. 9. Sample choosing. 10. Questionnaire application to a current working biology's professor sample. 11. Results analysis and interpretation. 12. Second sample choosing. 13. Depth interviews. 14. Result analysis and presentation. Results Findings in this study reveal that biology's professors use various explanation models in teaching biology, and previous ideas, which are transmitted to students as concepts, are contained in these models. Anthropomorphic (idea) that is characterized by showing mechanisms directly related to man and woman behaviour. Causal-functional (idea) that is referred to the "close causes" and the evolution (idea) that use the final causes. Descriptive-specific (idea) that focuses to a main element (oxygen, blood, food, etc.) from the biologic phenomena and the descriptive-tautological (idea) that use a universal enunciation to explain singular enunciations and vice versa. Telenomic (idea) that is when explanation about functions have a directional objective; and evolutive-teleologic (idea) that is when the finality with the natural selection process. The existence of previous ideas in explanation models, in addition to other factor in professor's training, is a factor that propitiates scarce comprehension of the meanings of scientific process in high school students in Mexico. References COLIN Wood, Robinson (1991). Young people's ideas about plants, Studies in Science Education, 19, USA DRIVER, R. and Easly J. (1978). Pupils and paradigms: A review of literature related to concept development in adolescent science students, Studies in Science Education (5), USA GAY, L.R. (1990). Educational Research (Third edition), Macmillan Publishing Company, USA HEWSON P. and M. Hewson (1987). Science teacher's conceptions of teaching: Implications for teacher education, International Journal of Science Education, Vo. 9, No. 4, USA KEEVS, John (1988). Educational Research, Methodology, and Measurement, Pergamon Press, Great Britain OSBORNE, R, and Freyberg P. (1985). "Children's science", in Osborne, R. and Freyberg P. Learning science: the implications of children's science, Hermann, England TREAGUST, D. (1991). A Case study of two exemplary biology teachers, Journal of research in science teaching, Vol. 28, No. 4 YIN, Din-Dan (1998), Identification of misconception in novice biology teacher and remedial strategies for improved, International Journal of Science Education, Vol, 20, No. 4, USA