An apparatus for determining the repellency of a chemical compound is described. Its action depends upon the insects' coming into contact with the compound deposited on a glass surface. The numbers of insects settling on treated and untreated glass plates at intervals over a period of 40 min. and under conditions that minimize varying disturbing factors are counted. The quotient obtained by dividing the total numbers on the untreated plates by the total numbers on the treated plates is called the ‘repellence quotient’, and must reach certain given values before significance is assumed and a compound is regarded as repellent.Typical results obtained with the apparatus are given and include the detailed determination of the repellency of butoxypolypropylene glycol (MW 8