The incorporation of mathematical modeling experiences into an undergraduate biology course is described. Detailed expositions of three models used to teach concepts of population ecology are presented, including introductions to major concepts, user instructions, trial data and problem sets. The models described are: 1) an exponential/logistic model of population growth; 2) a competition model; and 3) a model of the Lotka-Volterra predation equations for a system of two interacting predators. Some reasons for the increased use of mathematical modeling and systems theory are sketched, and the points are made that early exposure to mathematical modeling helps students to see the relationships between models and the real world, enables them to grasp the relevance of mathematics to biology, and gives them an introduction to important theoretical ideas. Finally, the ideal model is described as one which is consistent, clear in its directions to students, and rapid in response to input. (PB)