The harvestman Ilhaia cuspidata was studied in Curitiba, state of Paraná, Brazil. The site studied is a 30,000 m² forest remnant of Araucaria forest surrounded by houses. Harvestmen were sampled 21 times over 14 months (June 1997–August 1998); at intervals of 21–30 days. The population size was estimated by Fisher-Ford and Jolly methods, and did not vary considerably from autumn (June) to spring (November 1997). However, it increased rapidly (300 %) during late spring and summer (December– March). The lowest population size estimated was 1,429 adults/subadults, during the winter (June 1997) and, the highest was 14,445 during the autumn (April 1998). The recapture rates ranged from 16%–41%. The sex ratio observed in all sampling periods was 1:1. The density varied from 0.05 (winter) to 0.47 adult subadults/m² (autumn). The extremely different abundances observed between seasons could have been influenced by temperature. The immatures were observed all year, suggesting a continuous reproduction, but they were much more abundant during spring and summer. Ecological aspects including aggregation, individual movement and life span were also discussed.