ABSTRACTBacillus thuringiensisproduces toxins that target invertebrates, includingCaenorhabditis elegans. Virulence ofBacillusstrains is often highly specific, such thatB. thuringiensisstrain DB27 is highly pathogenic toC. elegansbut shows no virulence for another model nematode,Pristionchus pacificus. To uncover the underlying mechanisms of the differential responses of the two nematodes toB. thuringiensisDB27 and to reveal theC. elegansdefense mechanisms against this pathogen, we conducted a genetic screen forC. elegansmutants resistant toB. thuringiensisDB27. Here, we describe aB. thuringiensisDB27-resistantC. elegansmutant that is identical tonasp-1, which encodes theC. eleganshomolog of the nuclear-autoantigenic-sperm protein. Gene expression analysis indicated a substantial overlap between the genes downregulated in thenasp-1mutant and targets ofC. elegansdcr-1/Dicer, suggesting thatdcr-1is repressed innasp-1mutants, which was confirmed by quantitative PCR. Consistent with this, thenasp-1mutant exhibits RNA interference (RNAi) deficiency and reduced longevity similar to those of adcr-1mutant. Building on these surprising findings, we further explored a potential role fordcr-1inC. elegansinnate immunity. We show thatdcr-1mutant alleles deficient in microRNA (miRNA) processing, but not those deficient only in RNAi, are resistant toB. thuringiensisDB27. Furthermore,dcr-1overexpression rescues thenasp-1mutant's resistance, suggesting that repression ofdcr-1determines thenasp-1mutant's resistance. Additionally, we identified the collagen-encoding genecol-92as one of the downstream effectors ofnasp-1that play an important role in resistance to DB27. Taken together, these results uncover a previously unknown role for DCR-1/Dicer inC. elegansantibacterial immunity that is largely associated with miRNA processing.