  $UBV$ photometry is used to re-identify the OB associations which power the
two most luminous HII regions in M33, NGC 604 and NGC 595. There is a
significant difference (2-3 Myr) in the ages of the most recent star formation
episode in these two regions, while NGC 595 also has undergone a prior episode
of star formation (10-15 Myr ago). These data, combined with the presence of
molecular clouds in the heart of NGC 604, suggest that molecular clouds may
survive at least one intense episode of massive star formation. The star
formation efficiencies (mass of stars per mass of gas) of these two HII regions
are up to a factor of 3 larger than the average efficiency in the inner disk of
M33 or in Galactic molecular clouds, but are still only 2-5\%.
