This paper is the first in a series presenting CCD multicolor photometry for 145 HII regions , selected from 369 candidate regions from Boulesteix et al .
( 1974 ) , in the nearby spiral galaxy M33 .
The observations , which covered the whole area of M33 , were carried out by the Beijing Astronomical Observatory 60 / 90 cm Schmidt Telescope , in 13 intermediate-band filters , covering a range of wavelength from 3800 to 10000Å .
This provides a series of maps which can be converted to a multicolor map of M33 , in pixels of 1 \arcsec { \mbox { } \hskip { -4.267913 pt } . } 7 \times 1 \arcsec { \mbox { } \hskip { -4.267913 % pt } . } 7 .
Using aperture photometry we obtain the spectral energy distributions ( SEDs ) for these HII regions .
We also give their identification charts .
Using the relationship between the BATC intermediate-band system used for the observations and the UBVRI broad-band system , the magnitudes in the B and V bands are then derived .
Histograms of the magnitudes in V and in B - V are plotted , and the color-magnitude diagram is also given .
The distribution of magnitudes in the V band shows that the apparent magnitude of almost all the regions is brighter than 18 , corresponding to an absolute magnitude of - 6.62 for an assumed distance modulus of 24.62 , which corresponds to a single main sequence O5 star , while the distribution of color shows that the sample is blue , with a mode close to - 0.05 as would be expected from a range of typical young clusters .