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Stenocereus eruca is a clonal cactus with an extremely narrow distribution in Baja California, in which seedling recruitment has rarely been observed. Low seedling recruitment in clonal plants may be caused by low seed production as a consequence of pollinator limitation or if seed input is sufficient, by lack of favorable conditions or microsites for seedling establishment. In this paper, we study the reproductive ecology of S. eruca along four years in order to explore the proximate causes of the low seedling recruitment observed in its populations. Flowers are self-incompatible, secrete up to 200 mul of nectar with sugar concentration of 21-23% and are predominantly nocturnal, with little opportunities for diurnal visitors. Major flower visitors were sphingids (Hyles lineata and Erinnys ello) and an unidentified native bee. The proportion of flowers setting fruit (fruit set) was in general low, with values ranging from 0.03 to 0.15 among four populations from 1999 to 2002. Bees were observed visiting flowers during 1999, 2001 and 2002 while sphingids were observed visiting the flowers only during 2000. Pollination treatments showed evidence of pollinator limitation during 1999, 2001 and 2002, but not during 2000, when sphingids were observed. Overall, sphingids seem to be unreliable and likely to be the missing pollinator responsible for the low fruit set observed among populations of S. eruca.