Retinoic acid inducible gene (RIG-I)-mediated innate immunity plays a pivotal role in defence against virus infections. Previously we have shown that Sendai virus (SeV) defective interfering (DI) RNA functions as an exclusive and potent RIG-I ligand in DI-RNA-rich SeV-Cantell infected cells. To further understand how RIG-I is activated during SeV infection, we used a different interferon (IFN)-inducing SeV strain, recombinant SeVΔC, which, in contrast to SeV-Cantell is believed to stimulate IFN production due to the lack of the SeV IFN antagonist protein C. Surprisingly, we found that in SevΔC-infected cells, DI RNAs also functioned as an exclusive RIG-I ligand. Infections with wild-type SeV failed to generate any RIG-I-associated immunostimulatory RNA and this correlated with the lack of DI genomes in infected cells, as well as with the absence of cellular innate immune responses. Supplementation of the C protein in the context of SeVΔC infection led to a reduction in the number of DI RNAs, further supporting the potential role of the C protein as a negative regulator of DI generation and/or accumulation. Our findings indicate that limiting DI genome production is an important function of viral IFN antagonist proteins.