The Madagascar spider tortoise (Pyxis arachnoides), endemic to the threatened southern dry forests of southwest Madagascar, is thought to display highly seasonal variations in activity. As we increase our knowledge of the population dynamics of this species, it is becoming increasingly evident that there are probable wide ranging intra-population concentrations and annual activity levels across the eight remaining populations, spanning three subspecies and two intergrade populations. We used a sweep search monitoring method across five 1-ha survey plots within an area of the northern subspecies’ (P. a. brygooi) area of occupancy, close to the Baie des assassins. We monitored these plots on 11 evenly spaced occasions between november 2010 and february 2012. We documented significantly greater detection of tortoises during the wet season of 9.2 (sD ± 6.9) tortoises per/ha as compared to 0.8 (sD ± 0.4) during the dry season, with detection peaking during January 2012 with an average density of 19.8 tortoises per/ha. our results suggest that this population represents the highest known concentration of the species and a comparatively high proportion of juveniles within the population, compared to studies on other subspecies populations.