Oxygen microelectrodes were used to monitor oxygen concentration and rates of gross photosynthetic activity in Microcystis sp. scums which were formed and incubated under laboratory conditions. The depth of the photic layer, rate of photosynthesis, oxygen concentration and the location of the transition to anoxia in the scum depended on irradiance levels and colony size. Gross photosynthetic activity never extended below 2.5 mm depth in the scum. At high irradiance levels oxygen concentration in the upper 1.5 mm of the scum decreased and the oxygen concentration peak shifted to greater depth. Oxygen concentrations in scums composed of small colonies (< 55-mu-m) were higher than concentrations in large colonies scums (> 300-mu-m) but small colonies showed stronger indications of photoinhibition. In a natural scum small colonies are presumably shielded from inhibitory intensities by larger colonies which will dominate the upper layers. Accumulation of low-light adapted, smaller colonies in deeper layers likely yielded a second peak in photosynthetic activity. In order to systematically discuss scums and scum formation a distinction is made in three different scum types.