In 1949 when Hopkins reviewed the data then known about Mallophaga found on mammals, he recognized 288 species and subspecies of Mallophaga. Since then, 179 new species and subspecies of Mallophaga have been described, for a total of 467 valid species and subspecies.

There are at least 4,268 species of living mammals, and there is evidence that at least 3,400 species of mammals do not normally have Mallophaga. With better collecting, Mallophaga may be found on 512 species of mammals for which no current data exist. Mallophaga have now been recorded from 356 species of mammals.

Because Mallophaga are obligatory external parasites, their distribution is limited to that of their hosts. Most Mallophaga are host specific; some are less restricted in host selection, and a few are host subspecific. These variations in host specificity often aid mammalogists in their taxonomic studies of certain groups of mammals. Ten examples of Mallophaga host specificity and mammal relationships are discussed in detail.

Mallophaga recorded from 356 species of mammals are listed, and the list has been expanded to include subspecies of mammals in those instances in which the Mallophaga are host subspecific. Published records which are obvious misidentification, accidental or contamination have been omitted. Synonymies are given in Hopkins (1949).