The infection rate of an established colony of Culicoides variipennis to oral infection with bluetongue virus (BTV) type 4 was found to be highly variable and ranged from 0 to 51.6 per cent. This type of wide variation has previously only been reported to occur between individual populations of a species and not between samples from a single population. This must be taken into account when such colonies are used as reference standards for interpretation of field and laboratory data, particularly relating to studies of vector efficiency. Study of the response of individual susceptible C. variipennis to BTV infection demonstrated considerable variation in the level of virus multiplication that individual females are able to support. Virus concentrations varied from less than 1.1 to 5.1 log10 TCID50/fly up to 10 days post infection. 43.6 per cent of females contained less than 2.5 log10 TCID50 virus. It is suggested that such insects would have a mesenteron escape barrier to infection and would be incapable of transmitting BTV. The implications of these results for vector research are discussed.