The aim. To assess the relationship between body mass index (BMI) and gut bacteria in men and women with obesity. Materials and methods. The study included 56 overweight patients, divided into 2 groups. The first group consisted of 27 women (the average age was 62 ± 2.2 years), the second group — 29 men (the average age was 55 ± 9 years). The Quetelet index (kg / m²⁾ was calculated for all patients. To study the gut microbiome, the method of polymerase chain reaction in real time (RT-PCR) and metagenomic sequencing were used. DNA from feces was isolated using the Express-DNA-Bio DNA isolation kit (AlkorBio, Russia). To carry out RT-PCR, a set of reagents “Colonoflor-16” (“Alfalab”, Russia) was used. For microbiome sequencing, DNA libraries were prepared using the Illumina Nextera Sample Preparation Kit with DNA primers corresponding to the V3 — V4 regions of the 16S rRNA gene. The study of fecal samples was carried out using 16S rRNA gene sequencing on the Illumina platform (MiSeq sequencer). Results. It was revealed that a higher total number of bacteria, an increased content of Bacteroides fragilis group and Faecalibacterium prausnitzii, is statistically significantly more common in women than in men. Strong negative correlations were found between BMI and total bacterial mass, between BMI and the number of Bacteroides fragilis group among women with grade I obesity. In overweight men, a correlation was found between BMI and the Bacteroides fragilis group / Faecalibacterium prausnitzii ratio. Conclusions. The total number of bacteria, the content of Bacteroides fragilis group and Faecalibacterium prausnitzii in the gut of patients have statistically significant associations with BMI, and probably can affect the formation of metabolic disorders to a greater extent in women than in men. To clarify the identified trends and patterns in this pilot study, further study of the microbiome with a large number of patients and additional analyzes of the metagenome (16S rRNA) and metabolome, a transcriptome, allowing to control the expression of key metabolic enzymes, largely associated with the compositional features of the gut microbiocenosis, is required.