Bluetongue virus (BTV), a member of the Reoviridae family, is an insect-borne animal pathogen. Virus release from infected cells is predominantly by cell lysis, but some BTV particles are also released from the plasma membrane. The nonstructural protein NS3 has been implicated in this process. Using alternate initiator methionine residues, NS3 is expressed as a full-length protein and as a truncated variant that lacks the initial 13 residues, which, by yeast-two hybrid analyses, have been shown to interact with a cellular trafficking protein S100A10/p11. To understand the physiological significance of this interaction in virus-infected cells, we have used reverse genetics to investigate the roles of NS3 and NS3A in virus replication and localization in both mammalian and insect vector-derived cells. A virus expressing NS3 but not NS3A was able to propagate in and release from mammalian cells efficiently. However, growth of a mutant virus expressing only NS3A was severely attenuated, although protein expression, replication, double-stranded RNA (dsRNA) synthesis, and particle assembly in the cytoplasm were observed. Two of three single-amino-acid substitutions in the N-terminal 13 residues of NS3 showed phenotypically similar effects. Pulldown assay and confocal microscopy demonstrated a lack of interaction between NS3 and S100A10/p11 in mutants with poor replication. The role of NS3/NS3A was also assessed in insect cells where virus grew, albeit with a reduced titer. Notably, however, while wild-type particles were found within cytoplasmic vesicles in insect cells, mutant viruses were scattered throughout the cytoplasm and not confined to vesicles. These results provide support for a role for the extreme amino terminus of NS3 in the late stages of virus growth in mammalian cells, plausibly in egress. However, both NS3 and NS3A were required for efficient BTV growth in insect cells.