Regeneration of plants via tissue culture often results in a number of plants subsequently showing phenotypic or genotypic deviations from the parental type. This variation has been called somaclonal variation. In an analysis of regenerated Zea mays plants of the inbred line A188, high levels of phenotypic variation were observed. Subsequent analysis of these regenerated plants shows that a high proportion of the regenerants demonstrate significant alterations in the methylation status of both housekeeping and structural genes. These results are described and the theory of gene methylation is reviewed with regard to the differences that exist between plant and animal systems.Key words: 5-methylcytosine, 5-azacytidine, tissue culture, cereals, somaclonal variation.