An account is given of experiments designed to study the spread of myxomatosis in populations of rabbits living under natural conditions on a number of sites in the Eastern Riverine Plain of south-eastern Australia. In five different trials the disease spread from inoculated rabbits, but failed to gain momentum and died down within a few weeks of its introduction.

In December 1950, when the disease was persisting at low incidence on one test site and seemed to have died out in the others, an epizootic broke out in the neighbourhood which spread, in a few months, over the greater part of south-eastern Australia. The only factor, apart from climatic ones, which could account for this sudden change in the activity of the disease was the development of large populations of two rabbit-feeding mosquitoes, Anopheles annulipes and Culex annulirostris. A close correlation was demonstrated, on the flats bordering the Murray River, between the distribution of these insects and myxomatosis activity.

The author is indebted to Mr F. N. Ratcliffe, officer-in-charge, Mr J. le G. Brereton, Mr J. Calaby, and Dr R. Mykytowycz, all of the Wildlife Survey Section, Commonwealth Scientific and Industrial Research Organization, for assistance given during the course of the work and to Mr Ratcliffe, and Prof. F. Fenner of the Australian National University, for guidance in the presentation of the results.