SummaryAn account is given of the extent of parasitism ofSphaerularia bombiin bumble bees collected in Surrey from 1965 to 1968; data from Hertfordshire are also reported. From 1 to 42 parasites per host were recorded, and differences were noted in the levels of infection in various host species. In Surrey, approximately one quarter of overwintered queen bumble bees were affected byS. bombi, but infection appeared to be less severe in Hertfordshire. Variations in infection levels between host populations are discussed.