(200 words)There is scant evidence describing antimicrobial usage in companion animal primary-care veterinary practices in the UK. The use of antimicrobials in dogs and cats was quantified using data extracted from 374 veterinary practices participating in VetCompass. The frequency and quantity of systemic antimicrobial usage was described.Overall, 25% of 963,463 dogs and 21% of 594,812 cats seen at veterinary practices received at least one antimicrobial over the two-year period , and 42% of these animals were given repeated antimicrobials. The main agents used were amino-penicillin-types and cephalosporins. Of the AM events, 60% in dogs and 81% in cats were antimicrobials classified as critically important (CIAs) to human health by the World Health Organisation. CIAs of highest importance (fluoroquinolones, macrolides, third-generation cephalosporins) accounted for just over 6% and 34% of antimicrobials in dogs and cats, respectively. The total quantity of antimicrobials used within the study population was estimated to be 1473Kg for dogs and 58Kg for cats.This study has identified a high frequency of usage of antimicrobials in companion animal practice and for certain agents classified as of critical importance in human medicine. The study highlights the usefulness of veterinary practice electronic health records for studying AM usage.2