Copestylum alberlena, sp. nov. (Diptera: Syrphidae) was reared from larvae collected in decaying platyclades of Opuntia Mill. (Cactaceae) in the state of Veracruz (México). The third-instar larva, puparium, and adult morphology are described. Preliminary data about its life cycle and breeding behavior are included. This species is close to Copestylum tamaulipanum (Townsend), also breeding in Cactaceae, whose male genitalia are also described and compared with C. alberlena. The most important morphological differences between immature stages and adults of C. alberlena and C. tamaulipanum are illustrated and analyzed in a comparative table.