This paper reports on a case study with three dyads of high school students (age 14 years) each collaborating on a plant growth modeling task in the computer-supported educational environment "ModelsCreator". Following a qualitative line of research, the present study aims at highlighting the ways in which the collaborating students as well as the facilitator who supported them are engaged in the computer-based modeling "activity". The analysis is carried out with a two-level analytic tool that has been derived within the theoretical framework of "activity theory". Our results show that a wide range of modeling "operations" is activated in the context of the three major modeling "actions" of "analysis", "synthesis" and "testing-interpreting", which take place in the light of the facilitator-driven "action" of cognitive and technical support. Moreover, these actions are combined into "modeling units" of various forms which are repeated several times until the modeling process comes to an end. These many-fold repeats of the "modeling unit" appear to shape a pattern which characterizes the computer-supported shared "activity" as a whole.