The predatory behavior of adult females of Thermocyclops decipiens in relation to two cladoceran prey, Ceriodaphnia cornuta and Diaphanosoma spinulosum, was analyzed by laboratory observations. The number of attacks, captures, and ingestions were quantified. Probabilities of capture after attack (C/A) and ingestion after capture (I/C) were calculated as indicators of prey post-encounter vulnerability. T. decipiens attacked, captured and ingested both cladoceran species, but vulnerabilities of prey were relatively low. C/A was higher in C. cornuta than in D. spinulosum. Larger individuals of D. spinulosum showed higher C/A and I/C than smaller individuals but no effects of size were observed in C. cornuta. The ability of escape as well as the strength and shape of carapace, seem to partially explain differences in vulnerability among these species. Our results suggest that intensity of mechanical perturbations and other size-dependent factors, but not body size alone, could be important in determining post-encounter prey vulnerability among individuals of a same species.