This study was performed to estimate the prevalence of patent Parascaris equorum infections and determine the efficacy of ivermectin, pyrantel and fenbendazole against P. equorum infection in foals on farms in southern Australia. Foals aged >3 months on five farms in the south-western slopes region of New South Wales were used. Faeces were collected from each foal and foals with a P. equorum faecal egg count (FEC) of >100 eggs per gram (EPG) were used to measure anthelmintic efficacy using the FEC reduction (FECR) test, after random allocation to a control group or an ivermectin, pyrantel embonate or fenbendazole treatment group. Treatment was administered on day 0 and faeces were collected on day 14 and a FEC was performed. For determination of anthelmintic efficacy, FECRs and lower 95% confidence intervals (LCL) were calculated using previously described methods, based on individual or group FECRs. P. equorum populations were considered susceptible when FECR was >90% and LCL >90%, suspected resistant when FECR was FECR was 80-90% and LCL <90% and resistant when FECR was <80% and LCL <90%. A Poisson distribution quality control method was applied to the data to remove suspected erroneous FECR results. Prevalence of patent P. equorum infection was 58.3% (147/252 foals) and 89 foals on 5 farms were included in the FECR study. Resistance of P. equorum to =1 anthelmintic was present on all five farms prior to and on four farms after application of the quality control method. Two farms had evidence of multiple drug resistance. Ivermectin was effective and ineffective on two and three farms, respectively. Fenbendazole was effective on two farms, equivocal on one farm and ineffective on one farm. Pyrantel embonate was effective on three farms and ineffective on one farm. These data indicate that anthelmintic-resistant P. equorum populations are present on farms in Australia and multiple drug resistance may occur on individual farms.