504 The enrichment culture technique is one of the conventional approaches used to assess the composition of communities of methanotrophic bacteria in natural and anthropogenic ecosystems [1]. This technique has to be used because the number of cells of heterotrophic bacteria in samples usually exceeds the number of methanotrophs, significantly complicating their detection and identification. The use of enrichment cultures with methane as a selective growth substrate results in predominant development of the methanotrophic component of the microbial community and allows qualitative analysis of it. The question as to how the composition of the enrichment cultures formed under laboratory conditions corresponds to the composition of methanotrophic communities in the original sample is of fundamental importance. It is known that varying pH, the total salt content in the medium, or the incubation temperature results in obtaining enrichment cultures of different composition from the same samples [2‐4]. To date, it is commonly accepted that for the most adequate information about the composition of the methanotrophic community in situ to be obtained, the pH of the medium and the cultivation temperature should be approximated to those in the ecosystem studied. At the same time, not much attention is given to other conditions of enrichment culture setup, such as the presence and nature of a source of bound nitrogen in the medium and the culture aeration conditions. This investigation was undertaken to test the effect of these factors on the composition of laboratory methanotrophic enrichments and on how the latter corresponds to the methanotroph composition in the original sample. The direct molecular method for microorganism identification, the in situ hybridization with 16S rRNA-specific fluorescently labeled oligonucleotide probes (FISH), was used as the most objective referent method. Sphagnum peat was sampled from a 10‐20-cm layer of Bakcharskoe oligomesotrophic Sphagnum bog ( 56°51 ′ N, 82°51 ′ E) in Tomsk oblast in July 2006. The peat was homogenized by cutting with sterile scissors and mixed thoroughly. A 2-g portion of peat was fixed with 0.4% formaldehyde according to the method described earlier [5] for further analysis by FISH, and the remaining material was used for medium inoculation. Enrichment cultures were initiated in M2 medium (pH 5.0) [6] used in three modifications: (a) without a source of bound nitrogen; (b) with aNe 3 in an amount of 200 mg l –1 ; and (c) with (NH 4 ) 2 SO 4 in an amount of 200 mg l –1 . The media were dispensed in 20-ml por