The primary lesions produced in rabbits by the intradermal inoculation of the virulent French strain of myxoma virus were clearly distinguishable from those produced by the standard laboratory strain or by attenuated Australian field strains. Based on this fact a screening test was developed which allowed classification of large numbers of samples of virus into French or attenuated Australian types.

The virulent French type of virus was introduced into the wild rabbit population at Lake Urana by the inoculation of 242 rabbits over a period of 6 weeks, the attenuated Australian strain of virus having been recovered from a naturally infected rabbit at the beginning of the series of inoculations. A severe outbreak of myxomatosis occurred in which an estimated 70% of cases were caused by the French strain of virus. In spite of the early predominance of cases due to this introduced strain the majority of samples obtained in the latter half of the outbreak were of the attenuated Australian type, and this was probably the only strain which survived through the following winter and caused an intense epizootic in the spring.

A similar picture of early establishment of the artificially introduced virulent French strain, and its subsequent replacement by the naturally occurring attenuated Australian strain, was seen at two other study sites, North Urana Station and Merricumbene.

We are greatly indebted to colleagues on the staff of the Wildlife Survey Section at Albury, especially Mr M. P. Hines, for assistance in the field work, and to Mr P. Bentley, the manager of Cocketgedong Station, on which the experimental site is situated. We have profited from numerous discussions with Mr F. N. Ratcliffe, and are grateful to Dr G. S. Watson for assistance in the statistical treatment of the results.