Aquatic carnivorous plants of the genus Utricularia capture and utilise a wide range of small aquatic organisms. Most of the literature focuses on animals as prey. In this study, we investigate the occurrence of algae inside the traps of four species of bladderwort. We observed that algae of 45 genera form up to 80% of the total prey; algae were found frequently in traps without animal prey. The majority are coccal and trichal algae of the families Desmidiaceae and Zygnemataceae. The percentage of algae increases significantly with decreasing electric conductivity of the water (rS = −0.417; P = 0,000). Thus, algae are the most frequent prey in extremely soft waters. The percentage of algae did not differ significantly, not within the investigated Utricularia species or within the various study sites. However, the taxonomic composition of the algal prey showed highly significant differences between different sites. More than 90% of the trapped algae were killed and degraded by the bladders. The recent data allow for two alternative hypotheses: either algal prey supplements animal prey in oligotrophic waters, or the unprofitable trapping of algae is rather an additional stress factor for Utricularia and contributes to its limited distribution in some peat bogs.