A theory is developed for a Langmuir-type probe consisting of a collecting electrode placed at the stagnation point of a blunt body immersed in a supersonic partially ionized stream. It is shown that under certain conditions, the stagnation-point boundary layer equations and the probe sheath equations can be solved together to yield potential vs current relationships which permit the free stream ion and electron densities and temperatures to be measured by such a probe. It is shown also that the stagnation-point heat transfer will vary with probe potential, thus providing additional information useful in plasma jet diagnostics.