The epidemiology of ectoparasitic mites within the genus Psoroptes infesting domestic British livestock was investigated. The ear mite P. cuniculi was found to be common in sheep, goats, rabbits and probably horses. The body mite P. ovis was endemic within the National sheep flock, but was a sporadic, imported problem for cattle, with the last case caused by P. natalensis and not P. ovis. Equine mange (P. equi) has been eradicated from Britain. With the exception of P. ovis, mites from all other host species were not infestive to sheep. Comparative measurements of the male L4 outer opisthosomal setae revealed no statistical differences between ovine and bovine P. ovis and rabbit P. cuniculi, but P. cuniculi from the ears of sheep and goats, P. equi and P. natalensis were all statistically different from populations of sheep P. ovis and rabbit P. cuniculi. Setal lengths for goat and sheep ear mites were predominantly below 74.0 μm but mites isolated from the ears of rabbits fell into short ("typical ear canker") and long (generally "extra auricular") setal forms. Some "extra auricular" populations of P. cuniculi infesting rabbits were shown to contain sub-populations of P. cuniculi (non infestive to sheep) and P. ovis (infestive to sheep) and these populations could be selected for by ivermectin. A narrow band of bovine and ovine P. ovis divided the two forms of rabbit P. cuniculi. Subclinical, rapid growth and decline phases of disease were recorded for P. ovis infesting sheep, the duration of these phases varying with the virulence of the infesting population. Highly significant differences were observed between setal lengths for low, medium or high virulence population (73.6 μm, 80.2 and 90.5 μm, respectively). The scab mite P. ovis was also recorded in the ear canals of 38.6% of infested sheep, and prevalence was greater the higher the virulence of the infesting population. The majority of infestations were recorded in early rapid growth phase when lesions only covered 11.0 to 43.9% of the body and as far away as the mid back (and as early as 28 days after challenge) and only 22.6 % recorded in the decline phase (lesions 98.9% to 100.0% body cover), when the pinnae themselves could be infested. P. ovis and P. cuniculi are therefore synoxenous, occuring sympatrically on the same host (sheep) and may be syntopic (sharing the same habitat, ie the ear canal). P. ovis and P. cuniculi infesting sheep are not reproductively or ecologically isolated but are phenotypic variants of the same species. Thus populations high in P. cuniculi act almost as commensals of the ear canal and populations high in P. ovis act as highly pathogenic agents of mange. It is also suggested that the type species Psoroptes communis should be re-instated, with two variants infesting sheep, P. communis var ovis and P. communis var cuniculi.