This thesis investigates the neutrality of different mitochondrial DNA (mtDNA) haplotypes in the Hills Lake Domestic (HLD) strain brook charr (Salvelinus fontinalis). Two northern Ontario brook charr populations, derived from the HLD strain, were sampled, and the mtDNA distributions compared to the present HLD strain. Both populations had significantly more haplotype 1 fish than the HLD strain, suggesting differential survival of the mtDNA haplotypes. Two Algonquin Park lakes were stocked with HLD charr, and the survivors recaptured. There were no significant changes in haplotype frequencies over the summer. Thus, differential survival might occur in the wintertime. An experimental overwintering period with HLD brook charr found no evidence that fish with haplotype 1 have higher fitness than fish with other haplotypes. Fish with haplotype 1 had significantly greater condition factors at the start, but condition factors rapidly declined, so that on subsequent measuring dates there were no significant differences between haplotypes.