Two reservoirs, constructed for pollution control in thecatchmentarea of Lake Balaton, were studied. Both are hypertrophic duetohigh phosphorus loading, and both are rich in dissolved humicsubstances (colour in Pt units usually varies between50–150 Pt,mg l−1) as they were built on former wetland areas. Wemeasured planktonic primary production with 14Ctechnique, andbacterioplankton production using the[methyl-3H]thymidineincorporation method. The levels of dissolved organic carbon(DOC)and humic substances (as water colour) present wereinvestigated.Temperature, pH and light intensity at different depths wererecorded regularly. In both reservoirs studied thebacterioplanktonproduction was as high as the phytoplankton production and insomecases significantly exceeded it. Dissolved humic substancesprovedto be an important substrate for planktonic bacteria in thesewaterbodies.