ABSTRACT: In plants, 24 nucleotide small interfering RNAs (24-nt siRNAs) account for a large percentage of the total siRNA pool, and they play an important role in guiding plant-specific RNA-directed DNA methylation (RdDM), which transcriptionally silences transposon elements, transgenes, repetitive sequences and some endogenous genes. Several loci in plant genomes produce clusters of 24-nt RNAs, and these loci are receiving increasing attention from the research community. However, at present there is no bioinformatics resource dedicated to 24-nt siRNA loci and their derived 24-nt siRNAs. Thus, in this study, Pln24NT, a freely available web resource, was created to centralize 24-nt siRNA loci and 24-nt siRNA information, including fundamental locus information, expression profiles and annotation of transposon elements, from next-generation sequencing (NGS) data for 10 popular plant species. An intuitive web interface was also developed for convenient searching and browsing, and analytical tools were included to help users flexibly analyze their own siRNA NGS data. Pln24NT will help the plant research community to discover and characterize 24-nt siRNAs, and may prove useful for studying the roles of siRNA in RNA-directed DNA methylation in plants. AVAILABILITY AND IMPLEMENTATION: http://bioinformatics.caf.ac.cn/Pln24NT . CONTACT: [email protected]. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.