Intergeneric hybridization is not rare in Cactaceae, even between genera with very different pollination syndromes: thus, it appears that geographic barriers are more important than intrinsic reproductive barriers in this group. Here, we used nuclear microsatellite and chloroplast markers to examine reticulate evolution involving Haageocereus and Espostoa. Microsatellite data indicate that hybrids between Espostoa and Haageocereus are present and that hybridization has gone beyond the F1 generation, with sampled plants being either F2 or backcrosses, including several individuals that based on morphology were initially identified as Haageocereus. These populations meet the criteria of hybrid swarms. Together, plastid and nuclear markers indicate that Haageocereus, more abundant in the populations, was the pollen donor in all cases examined.