In a study with a new authentic sample of cis-allethrin, its toxicity relative to trans-allethrin and allethrin itself and the intensity of synergism of their 1: 10 mixtures with piperonyl butoxide were evaluated when applied as kerosene sprays on the Campbell turntable. The house fly (Musca domestica L.) was used as the test insect. cis-Allethrin was 0.546 as toxic as allethrin. The new estimation of the ratio for trans-allethrin and two previous estimations furnished a mean of 1.192 for that fraction. On the basis of these two evaluations the trans fraction comprised 70% and the cis fraction 30% of the sample of allethrin. This result agrees very well with reported results obtained by an infrared spectrophotometric method. On the basis of equivalents the piperonyl butoxide mixtures of trans-allethrin, cis-allethrin, and allethrin were, respectively, 2.94, 1.33, and 2.55 as toxic as allethrin alone. Synergism was demonstrated in all three mixtures, its intensity ratio for each being about 2.6 at the 50%, 1.8 at the 5%, and 3.7 at the 95% mortality levels. All sprays caused more than 90% knockdown of flies.