In seven enclosure experiments the short-term effects ofexposingBosmina longirostris, a small bodied cladoceran, to highlevels of fish predation were analyzed. The density of Bosmina did not change significantly in the experimentalenclosures, suggesting no direct fish impact on Bosminademography. However, detailed studies revealed strong fisheffectson some demographic and life history parameters of Bosmina.While the mean size of individual Bosmina decreased,reproduction enhanced (due to larger clutch sizes and a higherproportion of gravid females) in the enclosures with fishes.Thereis life history shift towards lower females sizes at a firstreproduction in the fish enclosures.The maintenance of Bosmina density in the fishenclosures maybe attributed to: (1) higher reproductive rate due to elevatedfoodlevel; (2) a life history shift towards smaller size reducingvulnerability to fish predation; (3) release from copepodpredationcaused by the presence of fish.