Until recently, it had generally been accepted that the life history of Ollulanus tricuspis was a complicated one involving an intermediate host, the mouse. Last year, however, I drew attention to the fact that the larval stages, both in the cat and in the mouse, believed by Leuckart to belong to Ollulanus, were really those of Ælurostrongylus, the lungworm of the cat. Subsequent work has enabled some of the details of the life history of Ollulanus to be elucidated—although by no means all—and these are considered in this paper.