We have identified a third member of the p38 group of MAP kinase termed p38 gamma. The cDNA for this MAP kinase encodes an 367 amino acid polypeptide that is slightly greater than 60% identical to p38 and p38 beta. Expression of its mRNA is primarily in muscle with no detectable expression in Northern blots using RNA from other tissues. In contrast to p38 and p38 beta, p38 gamma, fails to phosphorylate ATF-2 or MAPKAP kinase 2 but does like other MAP kinases phosphorylate MBP. In vivo kinase assays using protein extracts from skeletal muscle reveal that a 7-kDa protein is phosphorylated by p38 gamma but not by other members of this group of kinases. We suggest p38 gamma may have unique functions when compared with other members of the p38 group due to its restricted tissue expression and apparent substrate preferences.