The function of DNA methylation in higher plants was investigated by expression of a complementary DNA encoding a cytosine methyltransferase (MET1) from Arabidopsis thaliana as an antisense RNA in transgenic plants. This expression resulted in a 34 to 71 percent reduction in total genomic cytosine methylation. Loss of methylation was observed in both repetitive DNA and single-copy gene sequences. Developmental effects included altered heterochrony, changes in meristem identity and organ number, and female sterility. Cytosine demethylation prolonged both vegetative and reproductive phases of development. These findings implicate DNA methylation in establishing or maintaining epigenetic developmental states in the meristem.