Adaptation of bluetongue virus (BTV) to grow in mosquito cells (C6/36) resulted in overexpression of two non-structural proteins (NS3 and NS3a) in infected cells. These proteins also co-purified with BTV particles and were dissociated from the virions upon treatment with an anionic detergent. The expression was not host dependent, since back inoculation of the adapted virus into mammalian cell cultures also resulted in a significant overexpression of these proteins. The BTV-C6/36 produced smaller plaques in Vero cells compared with the parent strain. This is the first report which demonstrates a high level of NS3/NS3a expression in infected cells and subsequent release of infectious BTV particles into the supernatant.