Field observations on myxomatosis in wild rabbit populations of the Riverine Plain of south-eastern Australia, extending between August 1951 and March 1953, were reported. General observations on vector abundance and disease incidence were made over a large area. Detailed analysis of the population counts, age structure and immune status of the rabbit population, and the occurrence of insect vectors, were made at one site, Lake Urana.

The case-mortality rate in the first epizootic was in the neighbourhood of 99·8%, whereas in the second epizootic it was about 90 %, and many more chronically sick rabbits were observed in the later outbreaks. The virulence of eight strains of virus recovered from mosquitoes at Lake Urana in December 1952 and four strains recovered at Corowa at the same time was found to be somewhat reduced when they were compared with the original standard laboratory strain of virus. It is thought that this attenuation is the major reason for the observed change in the case-mortality rates.

The mosquitoes, Anopheles annulipes and Culex annulirostris were by far the most important insect vectors of myxomatosis in this region.

Thanks are due to Messrs F. N. Ratcliffe, B. V. Fennessy, and H. J. Frith of the Wildlife Survey Section of C.S.I.R.O. for helpful discussion and assistance during the field work and in the preparation of the paper. The technical assistance in the field of Messrs C. S. Hale and K. L. S. Harley is gratefully acknowledged, as is the generous help afforded by the landholders of Lake Urana, especially Mr J. Muldoon, and by Mr W. Quirk, rabbit inspector.