The purpose of this study was to assess the degree of ambivalence of the attitudes of twelfth-grade biology pupils in Israel towards interventions of man in nature. A questionnaire was designed in which the pupils were confronted with arguments of different levels of potential relevance (personal, global, and philosophical), three against and three in favor of each of eight “interventions of man in natural processes.” The pupil was requested (a) to rate each argument independently and (b) to “vote,” as if in a public committee, for or against each technology. The degree of ambivalence–-measured, for each item, by means of Kaplan's (1972) semantic differential half scales–-was the degree to which the pupils agreed to both positive and negative arguments. Although clear majorities of pupils voted in favor of all the “interventions,” three main patterns of responses were found: general agreement, indifference, and ambivalence. These patterns were characterized in terms of (a) the mean ratings of the arguments, (b) the differences between mean positive and negative “personal” ratings, (c) the percentages of favorable votes, and (d) the degrees of ambivalence. It was shown that pupils were able to appreciate arguments against a certain technology even in cases of strong personal feelings in favor of the technology (high ambivalence). The development of such an ability may be one of the main objectives of science education in a STS context.