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 plant is a subshrub which branches and forms a bushy clump up to 18 inches tall. The toothed leaves are up to 3.5 inches long. The yellow flowers bloom between March and November. They open in the morning and close in the afternoon. They fade orange or pink with age. The fruit is a cylindrical capsule.

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.