To explore the effects of constructivist learning environment on prospective teachers' opinions about "mathematics, department of mathematics, discrete mathematics, countable and uncountable infinity" taught under the subject of Cantorian Set Theory in discrete mathematics class, 60 first-year students in the Division of Mathematics Education at the Department of Science and Mathematics in Buca Education Faculty at Dokuz Eylul University were divided into two homogenous groups. In order to do this segmentation, Minimum Requirements Identification Test was developed and used by the researchers. This test includes concepts like "set", "correlation" and "function", which are required to understand Cantorian Set Theory. While the control group was taught by traditional methods, a teaching method based on a constructivist approach was applied to the experimental group. Data were gathered by an open-ended questionnaire administered to total 40 students, 20 from the each group. Collected data were evaluated through content analysis. In the end, despite the minor differences, no statistically significant difference was found between the opinions of control and experimental groups about mathematics (x[superscript 2] calculation=2.578, SD=3, p greater than 0.05), department of mathematics (x[superscript 2] calculation=3.185, SD=3, p greater than 0.05) and discrete mathematics (x[superscript 2] calculation=4.935, SD=3, p greater than 0.05) after the instruction. However, opinions about Cantorian Set Theory were significantly differentiated between experimental and control groups after the instruction (x[superscript 2] calculation=13.486, SD=2, p less than 0.05). (Contains 8 tablels.)