Recent work has demonstrated that microRNAs (miRNAs) are involved in critical biological processes by suppressing the translation of coding genes. Let-7is a critical regulator of developmental timing events at the larvaltoadult transition. MiRNAs in Paralichthys olivaceus were identified by Solexa sequencing. Ten miRNAs belonged to the Let-7d family (from Let-7a to Let-7j). The copy numbers of Let-7d were significantly higher than the other members of the Let-7family. Quantitative analysis revealed that the expression levels of Let-7d in the normal control group and the thyroid hormones (THs) treatment group had a common characteristic: expression gradually increased to a peak level, and then decreased to a low level. Let-7d levels in the THs treatment group were 50% less than those of the normal control group at each stage of development (P < 0.05). Let-7d was detected in all tissues examined, whereas at relatively higher levels in gill, heart and brain. Data of target predictions indicated that the 3′untranslated regions (3′UTRs) of 30 registered mRNAs had target sites matched with Let-7d. These genes were grouped into families: protein fold, cell proliferation, signal transduction, virus immune response and signaling pathways. The results of the present study suggested that Let-7d played an essential role in regulating the developmental events during metamorphosis in Paralichthys olivaceus.