Abstract  [Objectives] To investigate phase polyphenism, an apparent morphological plasticity of insects in which the expression of multiple phenotypic traits is directly regulated by population density, in the fall armyworm, Spodoptera frugiperta. [Methods] Fall armyworms were reared at either high, or low, densities and cuticular melanin, developmental duration, pupal weight and cannibalistic behavior, of individuals reared under different densities were compared. [Results] Larvae reared at high densities had smaller body size, more cuticular melanin and lighter pupal weights, than those reared at low densities. There was a higher incidence of cannibalism in larvae reared at low densities. [Conclusion] These results suggest that S. frugiperda larvae undergo density-based adaptations to minimize intraspecific competition, thereby enhancing individual survival. These findings further understanding of the population dynamics and outbreak characteristics of the fall armyworm, and the ecological consequences of cannibalism in this species.