Amplicon transgenes from potato virus X (PVX) are based on a modified version of the viral genome and are efficient activators of post-transcriptional gene silencing (PTGS). To determine whether PVX amplicons activate PTGS in Arabidopsis, we used constructs based on the genome of PVX carrying a green fluorescent protein (GFP) reporter gene. Our analysis of the transgene phenotype exploited previous observations indicating that PTGS is associated with short 25-nucleotide RNA species, transgene methylation, and homology-dependent virus resistance. We also used the ability of turnip mosaic virus to suppress gene silencing as a means of dissecting stages of the mechanism. The results showed that a PVX:GFP amplicon induces weak PTGS and that this PTGS was enhanced in the presence of a GFP reporter gene. Our interpretation of these data is that the PTGS induced by the amplicon was genetically determined and equivalent to the initiation stage of the PTGS mechanism. The PTGS induced by the combined amplicon and reporter gene was equivalent to the maintenance stage and was associated with an epigenetic conversion of the transgene. The distinction between genetic and epigenetic PTGS explains the well-characterized effects of transgene dosage on PTGS that have been previously interpreted in terms of RNA expression thresholds.