Pitahaya (Hylocereus spp) is a fruit that, due to the characteristics of its demand, both for fresh fruit and for industrial consumption, has detonated its development in various countries. However, it presents various self-incompatibility mechanisms that impact productivity. Therefore, the viability of pollen was analyzed during the anthesis (or flowering period)of two white-fleshed H. undatus cultivars, taking pollen samples for 13 hours, from the beginning of the anthesis in the afternoon, until it ends the following day. The method used was staining with tetrazolium salt (2,3,5-triphenyltetrazolium chloride) or TZ. The results showed that there is no viable pollen at the beginning of the anthesis. At 09:00 pm the viability began to grow slowly and after 12:00 am it doubled and the viability accelerated, marking the highest from 02:00 to 04:00 am. It was possible to observe the consistency of the pollen that begins with a phase I with humid pollen without viability, up to another with lower humidity with low viability (phase II). Phase III observed very granular mealy pollen, easy to handle precisely when the staining readings marked the viability of 93 % to 95 %. Subsequently, the viability decreases until it disappears when the floral structure begins to dehydrate.