We investigated the effects of juvenile Buenoa macrotibialis on the small bodied (<500 mm) zooplankton assemblage with which it naturally occurs. By monitoring 37– 46-liter enclosures with and without Buenoa over 9–12 d, we found that second and fourth instar Buenoa caused large declines in the copepod Tropocyclops extensus and the rotifer Hexarthra mira. Additionally, populations of several small rotifers increased in the presence of fourth instar Buenoa, probably because of release from Tropocyclops predation, competition, or both.