The objectives of this dissertation were to study the feasibility of using Drenchrite bioassay for the diagnosis of anthelmintic resistance in horse parasitic nematodes, determine the effects of moxidectin (an anthelmintic drug) selection on the population genetic diversity of Cylicocyclus nassatus, a common cyathostomin species and also to clone, sequence and characterize the orthologs of nematode glutamate-gated chloride channel (GluCl) genes from C. nassatus. Cyathostomin are the principal parasitic pathogen and most important intestinal nematodes of horses. Resistance against two of the three available classes of anthelmintics has been reported in cyathostomins. Although no clinical case report of resistance against avermectin-milbemycin (AM) class exists at present, it is highly probable that cyathostomin will develop resistance to this group of drugs in near future. First objective: Studies on validation of a larval development assay (LDA) for detection of anthelmintic resistance in horses in the current situation of anthelmintic resistance determined that a LDA cannot be recommended for use in the field in the present scenario of anthelmintic resistance in horses. Second objective: Selection using a subtherapeutic dose of moxidectin for a period of 29 months (21 doses) leads to a statistical decrease in population genetic diversity within C. nassatus as determined by Amplified Fragment Length Polymorphism (AFLP) for two of the three primer combinations tested. A high level of inherent genetic polymorphism in C. nassatus individuals was discovered in this study. Third objective: A homology based PCR app