the nuclear export receptor xpo1 supports primary mirna processing in c elegans and drosophila microrna mirna biogenesis proceeds from a primary transcript primirna through the premirna into the mature mirna here we identify a role of the caenorhabditis elegans nuclear export receptor xpo1 and the capbinding proteins cbp20ncbp2 and cbp80ncbp1 in this process the rnamediated interference of any of these genes causes retarded heterochronic phenotypes similar to those observed for animals with mutations in the let7 mirna or core mirna machinery genes moreover pre and mature mirnas become depleted whereas primary mirna transcripts accumulate an involvement of xpo1 in mirna biogenesis is conserved in drosophila in which knockdown of embargoedxpo1 or its chemical inhibition through leptomycin b causes primirna accumulation our findings demonstrate that xpo1emb promotes the primirnatopremirna processing and we propose that this function involves intranuclear transport andor nuclear export of primary mirnas