Thesis investigates the genetic diversity of various S. ‘boulardii’ strains using single nucleotide polymorphism (SNP) analysis of genomes sequenced in previous studies. In this work, 46 strains of the probiotic S. 'boulardii' were analysed with the aim to identify both common and unique SNPs and gain a better understanding of the genetic differences between these strains. Additionally, the genetic mutations of two distinct groups of isolates were examined: one from clinical sources and the other from commercial sources, to determine any significant differences and check if clinical group of samples was subjected to “in-host” selection. Through these analyses, we hope to identify any patterns or trends that suggest genetic adaptation or evolution.