The AMA1 sequence is an efficient plasmid replicator and transformation enhancer in Aspergillus nidulans. It comprises two long perfect inverted repeats (MATE elements) flanking a short, unique, central spacer. Subclone analysis indicates that the complete inverted duplication, but not the unique central spacer, is necessary for efficient plasmid replication. The smallest fragments able to affect transformation efficiency lie within the AT-rich portions of the inverted repeats. We demonstrate that two or more copies of the repeat in any relative orientation are able to perform the replicator function. A single copy of a MATE element increases transformation frequency to a modest extent but leads to multiple rearrangement, unstable integration or concatenation of vector molecules. Multimeric concatenates generated during this process are more stable mitotically, and when reisolated, transform the fungus at a much higher frequency than the original monomeric vector. Selection for multiple copies leads to the accumulation of multimeric products which resemble amplified DNA in various eukaryotic systems.