Since November 1906 I have had in the animal house attached to the Bacteriological Laboratory, a rather large female Macacus pileatus (No. 47). This monkey, which had been bought from a villager, had never been used for any experiment: it was kept in the same cage with a monkey inoculated with yas. In October 1907 the monkey, which so for had been in good health, began to lose appetite, and looked somewhat ill: nevertheless the animal continued to jump about in the cage and play with its compantion till a short time before death, on 5. xi. 07. The monkey never had any diarrhoea, and I thought it might have died from a peculiar form of malaria, extremely common in Ceylon monkeys.