;Mitogen-activated protein kinase (MAPK) cascades play important roles in signal transduction of extracellular stimuli in eukaryotes. However, stimulatory signals for plant MAPKs have not been well elucidated. Here, a cDNA clone, termed Oryza sativa MAPK4 (OsMAPK4), from rice encoding a protein that showed homology with the eukaryotic MAPKs was isolated. According to the phylogenetic analysis, OsMAPK4 belongs to subgroup IV MAPK in plants. OsMAPK4 transcripts were expressed strongly in mature leaves and weakly in young leaves and panicles. The gene was also differentially expressed in roots at different developmental stages. In addition, the mRNA level of OsMAPK4 was up-regulated under sugar starvation, high salinity and cold treatments. These results suggest that this OsMAPK4 functions not only in developmental programs but also in stress-signaling pathways.