Undergraduates with little training on molecular biology may find the technical level of the typical introductory restriction laboratory too challenging and have problems with mastering the underlying concepts and processes. "Blackboard electrophoresis" is an active learning exercise, which focuses student attention on the sequences and principles of DNA restriction. Students convert short strings of letters of A, C, G, and Ts into blackboard electrophoresis profiles analogous to gel electrophoresis of restriction digests. Students work in teams to i) invent short strings of letters representing polynucleotides; ii) identify and count the number of specific restriction sequences in each string; iii) fractionate the strings in restriction fragments and count their size; and iv) predict blackboard electrophoretic patterns in which fragments are represented by sizes. The exercise is inexpensive, since it does not require laboratory equipment or supplies and accommodates a plethora of introductory contexts that can be explored to bring in relevance to students. The exercise has been used with high school and university audiences with very positive outcomes. "Blackboard electrophoresis" is a valuable complement or alternative to other instructional approaches to teach restriction analysis and DNA diversity. (Contains 2 tables and 2 figures.)