Repeated within the intergenic spacers that separate adjacent ribosomal RNA (rRNA) genes in Xenopus laevis are several distinct sequence elements. These include transcription terminators, “region 0” repeats, “region 1” repeats, duplicated spacer promoters, and 42 bp enhancer elements that are embedded within 60 or 81 bp repeats. All have been reported to stimulate RNA polymerase I transcription from an adjacent gene promoter. A greater number of 42 bp enhancers per gene has been suggested to explain the preferential transcription of X. laevis rRNA genes in X. laevis x X. borealis hybrids, an epigenetic phenomenon known as nucleolar dominance. However, the possible contribution of regions 0/1 and/or spacer promoters to the preferential transcription of X. laevis (over X. borealis) rRNA genes has never been tested directly. In this study we systematically tested the various intergenic spacer elements for their contributions to promoter strength and nucleolar dominance-like competition in oocytes. In disagreement with a previous report, region 0 and region 1 repeats do not have significant enhancer activity, nor do they play a discernible role in X. laevis –X. borealis rRNA gene competition. Minigenes containing X. laevis spacer sequences are only dominant over minigenes having complete X. borealis spacers if a spacer promoter is located upstream of the 42 bp enhancers; X. laevis enhancers alone are not sufficient. These results provide additional evidence that spacer promoters together with adjacent enhancers form a functional activating unit in Xenopus oocytes.