My Science Tutor (MyST) is an intelligent tutoring system designed to improve science learning by elementary school students through conversational dialogs with a virtual science tutor in an interactive multimedia environment. Marni, a lifelike 3-D character, engages individual students in spoken dialogs following classroom investigations using the kit-based Full Option Science System program. MyST attempts to elicit self-expression from students; process their spoken explanations to assess understanding; and scaffold learning by asking open-ended questions accompanied by illustrations, animations, or interactive simulations related to the science concepts being learned. MyST uses automatic speech recognition, natural language processing, and dialog-modeling technologies to interpret student responses and manage the dialog. Sixteen 20-min tutorials were developed for each of 4 areas of science taught in 3rd, 4th, and 5th grades. During summative evaluation of the program, students received one-on-one tutoring via MyST or an expert human tutor following classroom instruction on the science topic, representing over 4.5 hr of tutoring across the 16 sessions. A quasi-experimental design was used to compare average learning gain for 3 groups: human tutoring, virtual tutoring, and no tutoring. Learning gain was measured using standardized assessments given to students in each condition before and after each science module. Results showed that students in both the human and virtual tutoring groups had significant learning gains relative to students in the control classrooms and that there were no significant differences in learning gains between students in the human and MyST human tutoring conditions. Both teachers and students gave high-positive survey ratings to MyST.