A concept map is a graph in which the nodes represent concepts, the lines between the nodes represent relations, and the labels on the lines represent the nature of the relations. Concept maps have been used to assess students' knowledge structures, especially in science education. Two concept mapping techniques, constructing a map and filling in a map that has been started were compared to see if the mapping techniques can be considered equivalent, whether the fill-in-the-map techniques are sensitive to the nodes selected to be completed, and whether the fill-in-the-map scores are sensitive to the linking lines selected to be filled-in. Participants were 152 high school chemistry students in 7 classes taught by 2 teachers. On three occasions students constructed or filled in maps as directed. The fill-in (skeleton) map scores were not sensitive to the sample of nodes or linking lines to be filled in. Fill-in-the-nodes and fill-in-the-lines are not equivalent forms of fill-in-the-map, but further research is needed to determine which of these forms provides more accurate information. Results suggest that both mapping techniques are tapping somewhat similar, but not identical, aspects of students' understanding. Construct-a-map scores more accurately reflect the differences across students' knowledge structures, and the relationship between scores from the multiple-choice test and both mapping techniques confirms that the mapping techniques are not equivalent. (Contains 6 tables and 15 references.) (SLD)