Leech is the most common ectoparasites on forest floor in tropical rainforests. However, there are few studies on its abundance and activity pattern, particularly in humid tropics where temperature and humidity on the forest floor seem suitable for leeches throughout year. In Endau Rompin National Park in peninsular Malaysia, we quantified the activity of brown leech (Haemadipsa zeylanica), and examined its diel and seasonal pattern, and its relationship with abiotic (e.g. humidity, and light intensity) and biotic (wildlife and human) factors using human-bait method. Diel activity pattern of leeches was not clear though the number of leeches found was slightly greater in nighttime (2000-0600 h) than in daytime (0800-1800 h). Seasonal activity pattern was relatively distinct; leeches were significantly more abundant in February and May (wetter season) than in August (drier season) as the abundance correlated positively with humidity and negatively with light intensity. Leeches were more abundant on trail than off-trail as the abundance was positively correlated with abundance of wild boar while negatively with abundance of Asian elephant and the distance from a river. Therefore, spatial pattern of leech population could be explained by abundance of a certain wildlife species and distance from water flow.