Computer simulations were employed by high school biology students in an attempt to enhance their problem solving skills. The simulations were administered under two conditions: (a) unguided discovery and (b) guided discovery. In addition, a control group received no simulations. To ascertain the effectiveness of the simulations in enhancing problem solving abilities, performance was compared on (a) subsequent unit pretests, (b) standardized tests measuring scientific thought processes, and (c) a standardized test of critical thinking. The results indicate that (a) the students using the simulations met the unit objectives at least as well as the control students, and (b) the students using the guided version of the simulations surpassed the other students on the subsequent simulation pretests, on the tests of scientific thinking, and on the test of critical thinking. The authors discuss the apparent usefulness of the programs in terms of the opportunities they provide students (a) to be actively involved in the learning process and (b) to repeatedly practice applying principles that would otherwise be practiced much less often.