[en] We have found that isoguanine (iG) can pair with thymine (iG · T) non-natural base, 5-methylisocytosine (iG · iCM) during template directed synthesis catalyzed by AMV reverse transcriptase. The ratio of these pairings is about 1:10, irrespectively which of the templates, poly(C,iG) or poly(I,iG) is used. This ratio corresponds to the ratio of 2-OH and 2-keto tautomers in monomer in aqueous solution and apparently it is not influenced by the template context. Our results indicate also that formation of the reverse transcriptase catalyzed base pairs between iG and A, G or C can occur only at low frequency. (author). 21 refs, 5 figs, 1 tab