1. Examination of monthly quotas of Apodemus sylvaticus, Clethrionomys glareolus, and Microtus hirtus, for parasites over a period of 21 months, enabled the seasonal occurrence of Trombicula autumnalis larvae to be determined.
2. All three species are infested during the summer and autumn months, and Clethrionomys throughout the following winter and spring also. Shrews were not parasitised.
3. The mites occupy different positions on different hosts.
4. Clethrionomys and rabbits (Oryctolagus) are of importance in maintaining the stock of harvest mites on some areas in the south of England.