Plant responses to crowding have been investigated extensively in stands of light-demanding species, but shade-tolerant species may react differently. In the present study, we investigated the effect of density on the mortality, size inequality, and biomass allocation of Alternanthera philoxeroides, a shade-tolerant invasive species. Stem fragments of A. philoxeroides were grown at either low or high densities (6 vs. 24 plants per pot) under three light levels (10%, 34%, and 100% full sun). After 8 weeks, survival was 31% lower in pots with a higher initial density. Both high density and low light levels reduced plant size substantially. Mean plant biomass ranged from 0.23 g in high-density and low-light pots to 4.41 g in low-density and high-light pots. There were no strong or significant effects of density or light level on size inequality of survivors. Most of the variation in allocation and morphology in response to light level and crowding were due to plant size and allometric growth, with little evidence of true plasticity. There was a small but significant increase in shoot allocation, in the direction predicted by optimal allocation theory, at low light levels. Our results show that intense competition need not be size asymmetric, and suggest that tolerance to low light levels involves a reduction in phenotypic plasticity. Responses of the invasive A. philoxeroides to crowding may be an example of an invasive plant‘s success in establishing dense stands of closely related individuals that are shade tolerant, cooperative, and follow a relatively fixed allometric trajectory with low plasticity.