The harvestman Phalangium opilio L. is an abundant predator in an array of habitats, including agricultural systems. In these systems, P. opilio has been noted to feed on pest species (e.g., Colorado potato beetle larvae, various aphid species, and lepidopteran eggs). Although the potential importance of P. opilio has been recognized, little is known about this harvestman’s development or reproductive biology. We investigated the influence of diet on development time, survival, reproduction, and female longevity. We compared the effects of two diets, Helicoverpa zea (Boddie) eggs and Aphis glycines Matsumura, on development of P. opilio because this predator is known to feed on those prey in soybean agroecosystems. Individuals fed A. glycines from emergence to adulthood suffered from higher mortality, longer development time, and smaller body size than those fed H. zea eggs. Four diet treatments were used in a separate study to test their effects on reproduction: standard laboratory diet (cornmeal, bacon, and H. zea eggs), H. zea eggs only, A. glycines only, and mixed diet (A. glycines and H. zea eggs). A diet of A. glycines alone yielded higher mortality, smaller body size, and lower fecundity than the standard laboratory diet. The presence of A. glycines in a monotypic or mixed diet lengthened the preoviposition period compared with diets without this prey and yielded a smaller first clutch and smaller average clutch sizes compared with the standard laboratory diet. A. glycines is detrimental to the growth and reproduction of this predator.