Discocactus zehntneri Britton & Rose subsp. petr-halfarii (Zachar) M.R. Santos & M.C. Machado (Cactaceae) is a Critically Endangered Brazilian cactus subspecies. There is no information on its population structure and phenological pattern. Thus, this study characterises population structure and determines the phenological pattern and germinative behaviour of this subspecies. We marked all individuals in the population and measured their diameter to create four age categories: seedling, juvenile, stem >5cm diameter without cephalium and reproductive plant. To carry out phenological assessments, we selected fifty plants. We monitored the plants monthly for two years. For each plant, we observed the number of reproductive structures and correlated them with climatic data. We found 1975 individuals grouped in subpopulations that grow in areas ranging from 800 to 2000m². Most individuals were seedlings and juveniles, and adult plants accounted for about 25% of the population. The flowering period started in November and extends to June, peaking in flower production between November and February, the months with higher rainfall and maximum temperatures. Fruiting was shorter, less intense and occurred between December and April. We observed only thirty-seven fruits in 163 flowers (22.7%) and recorded seed germination only in the presence of light (>90%). The results indicate that this cactus subspecies has a well-defined period of flowering and fruiting. Reproductive plants, fruits and seeds are few, which may affect the species maintenance in the environment.