Abstract: Field studies suggest that in phytotelm communities of Sarracenia purpurea, the absence of pitcher-plant specific larvae of the mosquito Wyeomyia smithii and the chironomid midge Metriocnemus knabi resulted in higher concentrations of Euglena. Conversely, a pitcher inquiline in which these larvae were present had lower concentrations of Euglena and algae, in general. In vitro experiments demonstrated that the Euglena-concentration with the absence of larvae, rose from near zero to 105 cells/mL after 14 day and by day-28, it exceeded 3 x 105 cells/mL. When only M. knabi larvae were present, the Euglena-concentration rose to 7 x 104 cells/mL by day-14 and to 2 x 105 cells/mL by day-28. When the larvae of the mosquito W. smithii were present, either by themselves or commensally with M. knabi, the Euglena-concentration remained below 104 cells/mL. Microscopic inspection of both M. knabi and W. smithii larvae revealed that, in the presence of Euglena, their guts turned green. We concluded that predation by W. smithii and M. knabi larvae suppress the growth of Euglena. These larvae prevent algae from overwhelming the pitcher-plant phytotelm and thus may play a critical role in maintaining the integrity of S. purpurea’s inquiline community.