The kinetic analysis of bacterial growth in zooplankton-free natural water in combination with new methods of bacterial counting (on slides or in a chamber) yielded a value of active bacterial biomass that exceeded values obtained by traditional methods (counting on filters) by 20 - 300-times. The reason for this discrepancy is the prevalence of filterable bacterial forms in bacterioplankton. A simplified colorimetric method for estimation of specific growth rate and the active biomass of bacterioplankton is suggested.