The turbulent jet in an infinite, parallel, and incompressible stream is considered. A coordinate-type perturbation expansion is developed and a method of series truncation applied to predict the downstream development of the mean flow. A constant eddy-viscosity coefficient across the jet is assumed, but its variation in the direction of streaming is obtained directly from the boundary-layer equations. The analysis differs considerably from any existing theory for jets in an external stream in that it is not an integral method and it does not require further assumptions about the character of the flow other than the one already mentioned. The solution derived compares favorably with the existing experimental results.