Introduction: Community effect is a phenomenon caused by cell–cell communication during myogenesis. In myogenic C2C12 cells in vitro, the confluent phase is needed for myogenesis induction. Methods: To examine the cell‐density effect, growth kinetics and myogenic differentiation were investigated in cells plated at four different cell densities. Results: We found that expression of a myogenic differentiation marker was high in a density‐dependent manner. At high density, where cell–cell contact was obvious, contact inhibition after the proliferation stage was accompanied by microarray findings demonstrating upregulation of negative regulating cell‐cycle markers, including CDKI p21 and the muscle differentiation markers MyoD and myogenin. Interestingly, developmentally regulated protein expression (drebrin) protein expression was also upregulated in a density‐dependent manner. Conclusions: These results suggest that contact inhibition after the proliferation stage may induce growth arrest via cell–cell communication through the expression of CDKI p21 and may be responsible for progressing cell fusion. Muscle Nerve 2011