Piwi proteins and their associated Piwi-interacting RNAs (piRNAs) are implicated in transposon silencing in the murine germline. There is currently little information on additional proteins in the murine Piwi complex and how they might regulate the entry of transcripts that accumulate as piRNAs in the Piwi ribonucleoprotein (piRNP). We isolated Mili-containing complexes from adult mouse testes and identified Tudor domain-containing protein 1 (Tdrd1) as a factor specifically associated with the Mili piRNP throughout spermatogenesis. Complex formation is promoted by the recognition of symmetrically dimethylated arginines at the N-terminus of Mili by the Tudor domains of Tdrd1. Similar to a Mili mutant, mice lacking Tdrd1 show derepression of L1 transposons accompanied by a loss of DNA methylation of their regulatory elements, and delocalization of Miwi2 from the nucleus to the cytoplasm. Finally, we show that Mili piRNPs devoid of Tdrd1 accept the entry of abundant cellular transcripts into the piRNA pathway and accumulate piRNAs with a profile that is drastically different from the wild-type. Our data suggests that Mili recruits Tdrd1 to ensure only the entry of transcripts that contribute to its normal piRNA pool. Immunoprecipitation of Tdrd1 or Mili from mouse testes isolated from wild-type or tdrd1 ko animals. Associated RNAs were sequenced in this study.