Thumba (Citrullus colocyntis) seed oil can be used for production of biodiesel. A comparative experimental study, carried out on a four cylinder, direct injection, water cooled diesel engine operating on diesel and biodiesel blends (B10, B20 and B30) of C. colocyntis and Jatropha curcas, showed that like J. curcas, biodiesel of C. colocyntis can be used as an alternative fuel with better performance and lower emissions compared with diesel.