  Much of our understanding of modern astrophysics rest on the notion that the
Initial Mass Function (IMF) is universal. Our observations of a sample of
HI-selected galaxies in the light of H-alpha and the far-ultraviolet (FUV)
challenge this result. The flux ratio H-alpha/FUV from these star formation
tracers shows strong correlations with surface-brightness in H-alpha and the R
band: Low Surface Brightness galaxies have lower H-alpha/FUV ratios compared to
High Surface Brightness galaxies as well as compared to expectations from
equilibrium models of constant star formation rate using commonly favored IMF
parameters. I argue against recent claims in the literature that attribute
these results to errors in the dust corrections, the micro-history of star
formation, sample issues or escaping ionizing photons. Instead, the most
plausible explanation for the correlations is the systematic variations of the
upper mass limit and/or the slope of the IMF. I present a plausible physical
scenario for producing the IMF variations, and suggest future research
directions.
