An elementary theory of aircraft trailing vortex decay is presented based on an assumed law for the variation of the mean eddy viscosity with distance from the wing. This law is based on the experimental data of Rose and Dee (1.963). The analysis gives results, as might be expected, in agreement with their data. The justification for the analysis must however be in doubt until more data are available covering a wide range of variables such as aircraft size, distance, incidence, etc.