An instrument for sorting time‐intervals into size‐classes is described. It discriminates between intervals which differ in length by only a few tenths of one percent. This makes it possible to study the time distribution of the impulses in Geiger‐Mu¨ller counters. Two argon counters, excited with gamma‐rays and operated at rates between 0.9 and 3.0 counts/sec., gave results in excellent agreement with fluctuation theory; but a hydrogen counter, operating at rates between 1.9 and 4.5 counts/sec., and excited either with gamma‐rays or with light, gave results so discrepant that they cannot reasonably be explained on the basis of statistical fluctuations or instrumental errors.