We examined variation in the floral morphology of Petrocoptis viscosa Rothm. (Caryophyllaceae), a plant endemic to the northwest Iberian Peninsula. First, we investigated whether peduncle length, calyx length and petal-limb length influence reproductive success (measured as seed production and seed weight). Second, we investigated whether floral morphology varies with position in the inflorescence and/or among populations. Peduncle length and calyx length showed no significant relationship with seed production or seed weight; however, petal-limb length was significantly and positively correlated with the number of seeds produced by the flower. All three morphometric characters, and the number of ovules per ovary, varied significantly both within positions in the inflorescence and between populations.

Key words: Caryophyllaceae, endemism, floral morphology, Petrocoptis viscosa, reproductive success