This study describes the effects of two selected electrophilic chemicals on the population growth kinetics of the freshwater ciliate Tetrahymena pyriformis (strain GL). The two chemicals selected, 3-butyn-2-one and trans-2-nonenal, each have a polarized, α,β-unsaturated moiety, which is associated with the electrophilic mechanism of Michael-type addition. T. pyriformis populations exposed to both electrophiles exhibited concentration dependent death of the initial inoculum within 1 to 2 h. Three-butyn-2-one which has a low 1-octanol-water partition coefficient (log Kow = -0.89) exhibited greater toxicity with death observed at 0.068 mg/L. Death was first observed in the morehydrophobic, trans-2-nonenal (log Kow = 3.16) at 4.21 mg/L. Death was followed by recovery of the population at the growth rates similar to control populations without chemical exposure.