The initial mass function (IMF) is very important to many aspects of astrophysics; it allows us to determine galactic properties such as the stellar mass of a galaxy, and is crucial in understanding star formation histories and galaxy evolution. Theoretically we expect the IMF to change with star forming conditions1. However, observations of stars in the Milky Way and Magellenic clouds show no significant variation2. This has led to heated debate over whether the IMF is universal or not. We are addressing this question by determining the form of the IMF in nearby galaxies. With the capabilities of the Hubble Space Telescope we can detect individual massive stars (M ≥ 2.5 Mʘ), allowing us to constrain the form of the upper-end IMF by statistically comparing simulated and observed colour-magnitude diagrams. We constrain the upper-mass limit, and the power-law slope for a variety of plausible star formation histories. We present results for the first object in our study, the outer-disk of NGC 2915, a blue compact dwarf galaxy at a distance of 4 Mpc. NGC 2915 is of interest due to its high mass-to-light ratio and low stellar density in its gas-rich outer-disk4.