Phalangium opilio L. is a polyphagous predator frequently found in agricultural habitats. Although the potential importance of P. opilio's feeding on pests has been recognized, little is known about its activity patterns or its within-plant distribution in crops. We determined diel activity patterns and microspatial distribution in small, fenced arenas in soybean fields. The fenced arenas allowed us to track known numbers of particular size categories of P. opilio for each 24 h trial. Phalangium opilio were separated into the following categories based on body size and sex: medium-sized nymphs, large-sized nymphs, adult females and adult males. Medium-sized nymphs occupy the bottom and middle portions of plants regardless of time of day; they remain still during the day, but they exhibit leg palpating behavior from 21:00–01:00 h. Large-sized nymphs rest in the bottom and middle portions of plants during the day, but they walk and palpate on the ground from 21:00–01:00 h. Adult females rest in the bottom, middle and top portions of plants during the day, and they walk and palpate on the ground from 21:00–01:00 h. Adult males remain stationary in the bottom, middle and top portions of plants during the day, but they walk on the ground from 21:00–04:00 h.