Years after the discovery that Dicer is a key enzyme in gene-silencing, the role of its helicase domain remains enigmatic. Here we show that this domain is critical for accumulation of certain endogenous small interfering RNAs (endo-siRNAs) in C. elegans. The domain is required for the production of the direct products of Dicer, or primary endo-siRNAs, and consequently, affects levels of downstream intermediates, the secondary endo-siRNAs. Consistent with the role of endo-siRNAs in silencing, their loss correlates with an increase in cognate mRNA levels. We find that the helicase domain of Dicer is not required for microRNA (miRNA) processing, or RNA interference following exposure to exogenous double-stranded RNA. Comparisons of wildtype and helicase-defective strains using deep-sequencing analyses show that the helicase domain is required by a subset of annotated endo-siRNAs, in particular, those associated with the slightly longer 26 nucleotide small RNA species containing a 5' guanosine. Overall design: We reintroduced either wildtype Dicer, or Dicer harboring a mutation (K39A) in it''s helicase domain, into dcr-1(ok247) mutant worms via transgene rescue. We then used high-throughput sequencing to compare levels of small RNAs present in each of these strains.