The immature stages ofAtrichopogon wirthiChan and Linley are found mostly on the lower surfaces of more mature leaves of the water lettuce,Pistia stratiotesL. Eggs, first instars, and pupae are found only on the leaf undersurface, while second to fourth instars may occur also on top of the leaf. Eggs are laid mostly in the large undersurface grooves 1–11 cm above the wet–dry line (most from 5–10 cm). First instars are found close to the eggs, on the distal third of the leaf. Second instars are more widely distributed, with the majority situated around or just above the wet–dry line, and with occasional individuals 1–10 cm above it. Third and fourth instars also occur mainly around the wet–dry line (0.5 cm below to 4 cm above). Pupae are distributed more distally, 3–9 cm above the wet–dry line.Pistiaplants examined between July and October 1987 yielded an average of 28.3 unhatched (presumably viable) eggs, 4.5 first, 2.5 second, 1.8 third, and 2.4 fourth instars, 2.3 pupae, and 2.8 pupal exuviae per plant. Certain inferences concerning the life history and behavior ofA. wirthiare discussed