Abstract
SUMMARY: The expression of the virulence character in the virulent line (YM) ofPlasmodium yoelii yoeliiwas investigated. The level of virulence was measured by counting the parasitaemia of the mature red blood cells. Several sub-clones were isolated from the virulent line YM and each was tested for its level of virulence. Out of 10 sub-clones 1 showed a marked decrease in virulence. However, transmission of this sub-clone through mosquitoes fully restored its virulence. A clone isolated from the progeny of a cross between mild and virulent parents had an intermediate level of virulence. A sub-clone isolated from this intermediate virulent line exhibited greatly reduced virulence. Mosquito transmission of this sub-clone also restored its virulence to a level comparable with the virulent line YM.