Monte Carlo calculations of ion collection by a rocket-borne mass spectrometer have been completed for the collisionless and transition flow. In the collisionless case, computations have been completed for the following range of parameters: Debye numbers of 0.01 to 1000, speed ratios of 0.1 to 3.1, and nondimensional potentials of -10 to -200. For a speed ratio of 1.5 and a potential of -200 the ion flux coefficient at the stagnation point decreases from approximately 40 at a Debye number of 1000 to 3 at a Debye number of 0.01. The stagnation flux varied from 11 at a speed ratio of 3.1 to 260 at a speed ratio of 0.1 for a Debye number of 1000 and potential of -200. The effect of varying the plate potential was found to be a linear variation of flux coefficient with collecting potential. Calculations made in the transition regime show that the minimum altitude at which the free molecule flow assumption is valid is approximately 120 km. A calculation at 70 km corresponding to a Knudsen number of 0.007 indicates that the flow field can be characterized as being near continuum but is such that rarefaction effects are still important. An investigation of the effects of collision cross section shows that the ion flux for an ion-neutral collision corss section 7 times larger than the neutral cross section is about half the flux for equal cross sections. Comparison of the calculations with flight data shows that the numerical results are higher by a factor of 3 or 4 than the measured values.