Mobile serious games are increasingly utilized as educational tools in elementary schools, and a considerable body of research has focused on evaluating such games' educational effectiveness. However, such work has generally ignored learning processes, and especially how knowledge is constructed. Given the important role of knowledge construction in various educational settings, this study examines it in the context of 83 elementary schoolers' mobile serious game-playing behaviors. Lag-sequential analysis of the participants' observed behavioral patterns, and of differences in such patterns between two performance subgroups (i.e., students with high vs. low academic performance), yielded two main findings. First, all these young learners exhibited knowledge construction, and moved smoothly from lower to higher phases of it in the mobile environment; and second, the high-performing group attained a deeper level of knowledge construction through the negotiation of meaning than the low-performing group did. Some theoretical and practical implications of these results are also discussed.