The aim of our research is to make a convenient and accurate resonant cavity dielectric measuring system for samples in the shape of a small rod or filament. The use of a circular TMOlO mode for rod samples is well known (1) but is not always so convenient because the sample changes the cavity frequency, which may necessitate re-matching of several parts of the system. This mode is, however, very useful at frequencies of the order of 1000 Mc, or when the rod is very small. We have increased the utility of this mode by giving a hole correction for the hole through which the sample is inserted (2).