We studied the relationship between mitotic regimen and incidence of micronuclei in liver and bone marrow cells of tailless amphibians under the effect of genotoxic carcinogen N-nitroso-N-methylcarbamide. Micronuclei appeared in amphibian somatic cells normally and after mutagenic exposure as a result of abnormal mitosis and of "interphase chromatin diminution". This latter variant was most incident in lymphoid cells, presenting as "caudate nuclei", which should be taken into consideration when interpreting findings of micronucleus analysis.