Background: A reduction in the Egg Reappearance Period (ERP) has been suggested to be an early indication of emerging anthelmintic resistance in strongyles. Objective:To measure the strongyle ERP following moxidectin treatment of horses in the southeast of England. Study Design: Prospective study. Methods: Horses with a faecal egg count (FEC) of > 400 strongyle eggs per gram (EPG) in a routine screening sample were enrolled into the study. Moxidectin (400 mcg/kg) was administered per os and FEC tests repeated every 2 weeks for 16 weeks. Results: Forty-eight horses completed the study. The mean EPG prior to treatment was 1047 (range 375 – 2137 EPG). In all but two horses, FEC was 0 EPG 2 weeks after moxidectin administration. In the remaining two, the FECs were 12.5 EPG (97.8-98.3% reductions compared to pre-treatment FEC). At 4 weeks post-treatment, 6 horses had positive FECs (96.6-99.2% reductions). At 6 weeks, 11 horses had positive FECs (83.8% reduction in one horse; >90% reduction in 10). At 8 weeks, 21 horses exhibited positive FECs (<90% reduction in 2). At 10 weeks, 27 horses had positive FECs (<90% in 6). At 12 weeks, 31 horses had positive FECs (<90% reduction in 11). At 14 weeks, 34 horses had positive FECs (<90% reduction in 13). At 16 weeks, 38 horses had positive FECs (<90% reduction in 17). Limitations: Weights of some horses were estimated using weigh tapes rather than a weighbridge. Dosing of the horses with moxidectin was carried out by owners. Conclusions: The results indicated acceptable efficacy of moxidectin at 14 days after treatment; however, the ERP pattern measured across the group suggest that this anthelmintic has a considerably shorter suppressive effect on strongyle egg shedding than measured when it was first introduced ( >13 weeks and up to 24 weeks).