MicroRNAs play important roles in regulating development at both transcriptional and posttranscriptional levels. Here, we report 29 microRNAs from mouse testis that are differentially expressed as the prepubertal testis differentiates to the adult testis. Using computational analyses to identify potential microRNA target mRNAs, we identify several possible male germ cell target mRNAs. One highly conserved sequence in the 3′-untranslated region (UTR) of transition protein 2 (Tnp2) mRNA, a testis-specific and posttranscriptionally regulated mRNA in postmeiotic germ cells, is complementary to Mirn122a. Mirn122ais enriched in late-stage male germ cells and is predominantly on polysomes. Mirn122a,but not another noncomplementary microRNA, inhibits the activity of a luciferase reporter construct containing the 3′-UTR of Tnp2.Site-directed mutations of Mirn122aindicate that base pairing of the 5′-region of Mirn122ato its complementary site in the 3′-UTR of Tnp2mRNA is essential for the downregulation of luciferase activity. Real-time reverse transcription-polymerase chain reaction and ribonuclease protection assays reveal that the Mirn122a-directed decrease of the Tnp2reporter gene activity results from mRNA cleavage. We propose that specific microRNAs, such as Mirn122a,could be involved in the posttranscriptional regulation of mRNAs such as Tnp2in the mammalian testis.