  We present Stroemgren CCD photometry for the two galactic globular clusters
M55 (NGC 6809) and M22 (NGC 6656). We find average Stroemgren metallicities of
-1.71 dex for M55 and -1.62 dex for M22. The determination of metal abundances
in cluster giants with the Stroemgren m1 index in comparison with spectroscopic
data from Briley et al. (1993) and Norris & Freeman (1982, 1983) shows that M55
and M22 have different distributions of cyanogen strengths. In M55, no CN
abundance variations are visible among the giant-branch stars. In striking
contrast, a large dispersion of cyanogen strengths is seen in M22. For M22 we
find patchily distributed variations in the foreground reddening of
E(B-V)=0.07, which explain the colour dispersion among the giant-branch stars.
There is no evidence for a spread in iron within M22 since the variations in m1
are dominated by the large range in CN abundances, as already found by
Anthony-Twarog et al. (1995). The difference between M55 and M22 may resemble
the difference in integral CN band strength between M31 globular clusters and
the galactic system. The colour-magnitude diagram of M55 shows the presence of
a population of 56 blue-straggler stars that are more centrally concentrated
than the red giant-branch stars.
