  We have measured star formation histories (SFHs) and stellar masses of
galaxies detected by the Balloon-borne Large Aperture Sub-millimetre Telescope
(BLAST) over approximately 9 square degrees centred on the Chandra Deep Field
South. We have applied the recently developed SFH reconstruction method of Dye
et al. to optical, near-infrared and mid-infrared photometry of 92 BLAST
galaxies. We find significant differences between the SFHs of low mass (<10^11
M_sol) and high mass (>10^11 M_sol) galaxies. On average, low mass galaxies
exhibit a dominant late burst of star formation which creates a large fraction
of their stellar mass. Conversely, high mass systems tend to have a significant
amount of stellar mass that formed much earlier. We also find that the high
mass SFHs evolve more strongly than the low mass SFHs. These findings are
consistent with the phenomenon of downsizing observed in optically selected
samples of galaxies.
