A previous study showed that unlike X. laevis, X. borealis contains a predominant or major histone gene cluster, which contains 70% of the 80-90 copies of the H4 genes in the genome (Turner & Woodland 1983 (Nucl. Acids Res.11 978)). This prompted more detailed analysis of histone gene arrangements in X. borealis, which is the subject of this thesis. Clones containing histone genes were isolated from a library prepared for this purpose. Analysis of these clones indicated one class containing the major cluster, and a second, minor class which appeared to be cloned at high frequency. Representative members of each class were characterised in detail. Major cluster clone XbHW302 was restriction site mapped, and certain regions sequenced. Microinjection of the major cluster clones in to Xenopus oocytes, confirmed that these genes were functional. Attention was then turned to the chromosomal organisation of the major cluster. A 'chromosome walk' experiment allowed the isolation of clones indicating a tandem of arrangement of clusters on the chromosome. This allowed the complete 'repeat' to be mapped in detail, further studies confirmed that the majority of clusters are tandemly repeated in the genome. The minor cluster clones were analysed in a fashion similar to the major class. Restriction site mapping and DMA sequencing of clone XbHW61 allowed the location polarity, and identity of an H3, H4, and an HI gene to be determined. Microinjection analysis again showed the genes were functional. Extensive DNA sequence comparisons between various Xenopus histone gene clusters were undertaken. The picture of histone gene cluster structure in X. borealis that emerges from these studies, is compared at the nucleotide, gene, and chromosomal levels with X. laevis. A discussion of how the differences between such closely related species could have arisen is also included.