Replication of co-injected supercoiled DNA molecules in fertilized Xenopus eggs was monitored through the blastula stage of development. The extent of replication, as measured by 32P-dTMP incorporation into form I DNA, was directly proportional to the number of molecules, rather than the size, of the plasmid injected. Although only a small fraction of molecules of either template was replicated, incorporation was predominantly into full length daughter molecules. Over at least a 20-fold concentration range of microinjected DNA, injection of equal masses of DNA resulted in greater incorporation into the smaller form I DNA present in molar excess. The extent of incorporation into supercoiled DNA for a particular plasmid was apparently independent of the concentration of a second, co-injected plasmid. The relative extents of replication of co-injected supercoiled templates could be altered simply by changing the molar ratios of the templates.