A population of the citrus nematode Tylenchulus semipenetrans Cobb (Tylenchida: Tylenchulidae) associated to a Pasteuria sp. (Bacillales: Bacillaceae) was studied in a naturally infested field. In a first population dynamics study, prevalence never exceed 50 % and showed a density-dependent relationship with the host at 2–3 months time lags, with a spring sharp increase. In a spatial sampling study, both organisms resulted uniformly distributed and matched the observed relationship. In top (10 cm) soil the adult females were lower than in deeper layers, whereas the nematodes with adhering endospores were higher. Anderson and May’s Model G applied showed that 50–400 endospores per cc of soil can sustain stable regulatory cycles. The nematode and bacterium populations were found in the field 12 years later with declining densities, below the calculated threshold needed to maintain the parasite infection. Data confirmed the bacterium potential in nematode regulation and showed that host numbers affect prevalence changes in time. © 2015 International Organization for Biological Control (IOBC)