Allelopathy is a process by which a plant releases chemicals that inhibit competitors. Recent phytochemical studies have reported the presence of phenolic molecules in some species of Pilosocereus, which could be indicative of allelopathic effect. The aim of this study was to assess the allelopathic potential of the fruit pulp extract of Pilosocereus gounellei on the germination of seeds of Lactuca sativa and P. gounellei. We tested the effects of pulp extract at four different concentrations. We measured the osmotic potential of the extract at the different concentrations, in order to assess its effect on seed germination. The extract negatively affected the germination parameters of both species and effect was more significant at higher extract concentrations. The influence of the osmotic potential on the germination parameters was not significant for most of the studied parameters. We believe that our results are indicative that the fruit pulp extract of P. gounellei contains allelochemicals that delayed or inhibited seed germination of not only L. sativa, but also of seeds of their own species (autoallelopathy). Further studies are necessary to clarify the inhibition mechanisms, and how widespread the allelopathy is across other cactus species.