Rootless carnivorous plants of the genus Utricularia are important components of many standing waters worldwide, as well as suitable model organisms for studying plant-microbe interactions. In this study, an investigation was made of the importance of microbial dinitrogen (N-2) fixation in the N acquisition of four aquatic Utricularia species and another aquatic carnivorous plant, Aldrovanda vesiculosa. 16S rRNA amplicon sequencing was used to assess the presence of micro-organisms with known ability to fix N-2. Next-generation sequencing provided information on the expression of N-2 fixation-associated genes. N-2 fixation rates were measured following N-15(2)-labelling and were used to calculate the plant assimilation rate of microbially fixed N-2. Utricularia traps were confirmed as primary sites of N-2 fixation, with up to 16 % of the plant-associated microbial community consisting of bacteria capable of fixing N-2. Of these, rhizobia were the most abundant group. Nitrogen fixation rates increased with increasing shoot age, but never exceeded 1 center dot 3 mu mol N g(-1) d. mass d(-1). Plant assimilation rates of fixed N-2 were detectable and significant, but this fraction formed less than 1 % of daily plant N gain. Although trap fluid provides conditions favourable for microbial N-2 fixation, levels of nif gene transcription comprised