Different genera and groups of heterotrophic planktonic bacteria of the littoral zone of the lake Jeziorak produced different B-group vitamins. Most numerous among the planktonic bacteria of this zone of the lake were biotin producers and least numerous were organisms synthesizing riboflavin and nicotinic acid. Most bacteria produced one or two vitamins. Three or more vitamins were produced by only a few strains. In autumn no strain produced all five vitamins studied in this work.