In the dairy industry, bovine mastitis caused by ' Staphylococcus aureus' is a major production issue as mastitic cows produce milk which is reduced in both quality and quantity. This work examined the potential uses of biological control agents for the treatment and prevention of bovine mastitis caused by 'S. aureus'. Bacteriophage therapy was studied both 'in vitro' and 'in vivo' in order gain an understanding of the dynamics and efficacy of this approach to mastitis treatment. The interaction of the lytic bacteriophage K and ' S. aureus' strain Newbould 305 was studied 'in vitro' in raw bovine whey. Whey proteins were found to inhibit phage attachment to the 'S. aureus' cell surface and thereby hindered lysis. Novel broad host-range phages related to phage K were also isolated from the environment. A mutant phage, designated Stau6a, exhibited an extended host-range and lytic activity comparable to that of phage K 'in vitro'. The ability of phage K to treat naturally occurring, subclinical 'S. aureus' mastitis during lactation was evaluated in a placebo-controlled, multi-site trial containing 24 lactating Holstein cows. The cure rate was three out of 18 quarters (16.7°,%) in the phage-treated group, while none of the 20 saline-treated quarters were cured. This difference was not statistically significant. This lack of efficacy was due to an apparent combination of ' in vivo' phage inactivation and inhibition of phage activity by milk proteins. For the prevention of bovine mastitis, the bacterial flora inhabiting the teat canals of lactating dairy and beef cattle was studied using 16S rDNA-based methods. The teat canal is the primary route through which pathogens are introduced into the mammary gland, thus its microbial content may influence the prevalence of mastitis. A highly diverse population of 16S rDNA sequences was recovered from both the dairy and beef herd samples. The results suggest that the microorganisms present in the bovine teat canal are more diverse than previously described using culture-based methods; the ecology of this site may be complex. Prevention of mastitis by colonization of the teat canal with beneficial microorganisms may yield some benefit, and is an area which requires further study.