Biology of amaranth stem-boring weevil, Hypolixus pica (Fabricius) on red root pigweed, Amaranthus retroflexus L. was investigated under natural conditions in a citrus orchard in Dezful. Each red root pigweed was covered by a wooden cage of 50×50×100cm dimentions screened by muslin. Three to four adult females and males were introduced into each cage. After 3-4 days past the weevils were removed and the biology of the insect in its immature stages investigated. The insect bore three generations. Under natural conditions the duration of egg and pupa of the second generation were 3 and 8.86±0.84 days and the duration of egg and pupa of the third generation 3.55±0.49 and 10.33±0.47 days, respectively. The developmental time of the second and third generations lasted 44.68±4.01 and 50.88±4.67 days, respectively. The weevil was found to overwinter as either its last larval instar (fourth) or as adults. Females laid eggs in stems covering them by a black secretion. The larvae moved downwards, tunneling the stem. The adult stage made a hole above the gall and left the stem. The adult stage insect damaged the leaves and the seeds of A. retroflexus as well.