The Mythic 18 is a fully automated haematology bench-top analyser using impedance technology for a complete blood count (CBC) and a 3-part white blood cell (WBC) differential. The purpose of this study was to evaluate the Mythic for accuracy, precision, linearity, carry-over, stability and usability under practice conditions. EDTAblood samples from 122 dogs, 140 cats and 123 horses were analysed with the Mythic and reference methods (Sysmex XT-2000iV, manual haematocrit and microscopic WBC differentiation). Red blood cell parameters showed excellent correlation and small biases. Total WBC count correlated excellently in canine and equine and very well in feline samples. In 23 feline specimens with platelet aggregates, the Mythic overestimated WBC counts. In all three species, absolute granulocyte counts correlated excellently. Equine lymphocyte counts showed good correlation whereas canine and feline lymphocyte counts correlated poorly. Feline platelets showed good correlation with a negative bias. The instrument showed good to excellent precision and performed excellently for the CBC count parameters in all investigated species. The whole 3-part differential was found to be accurate in horses. In dogs and cats absolute granulocyte counts were reliable. As with all impedance based haematological instruments, evaluation of a blood smear is absolutely indicated to check for the presence of platelet aggregates, to verify WBC differentiation and to identify possible pathologies.