Calylophus serrulatus is a species of flowering plant in the Onagraceae known by the common name yellow sundrops. Other common names include halfshrub sundrop, serrate-leaved evening primrose, shrubby evening primrose, plains yellow primrose, and halfleaf sundrop. It is native to central North America, including central Canada and the central United States. This drought- and heat-tolerant species grows in many types of substrate, including caliche, limestone, and gypsum. The leaves turn so that their edges face the sun, an adaptation to hot conditions.