Conceptions of mentoring held by six mentors and six beginning science teachers in an alternative certification program were explored qualitatively by means of case studies and phenomenography. Interviews with the six mentors and six beginning teachers produced 379 statements that were grouped into six conceptual categories. The categories of "apprenticeship, personal support," and "colearning" revealed the variation in how mentors and beginning teachers conceptualized school-based mentoring. The conceptions functioned as referents for their mentoring practice. Mentoring as apprenticeship was the dominant conception among both the beginning teachers and mentors. The findings of this study imply that conceptions of mentoring held by mentors and beginning science teachers should be considered when organizing the school-based component of alternative certification programs.