By use of a two-choice laboratory bioassay, olfactory response to beetle and frass volatiles was assessed inOryzaephilus surinamensis(L.) andO. mercator(Fauvel) of known age and mixed sex from cultures held at various adult population densities. Four- to 6-week-postemergentO. surinamensisfrom dense laboratory cultures (~40,000 beetles per kg of medium) did not respond to, or were repelled by, Porapak Q-captured volatiles from beetles, frass, rolled oats, and brewer's yeast. When culture density was lowered to 1,000 beetles per kg or when beetles>2 months posteclosion were tested, positive olfactory response to test volatiles progressively developed. Recently eclosedO. surinamensisfrom low-density cultures exhibited a period of negative attraction or no significant response to test volatiles, followed by a gradual increase in positive response. In contrast, forO. mercatormaintained in moderately crowded cultures (10,000 beetles per kg), positive olfactory response to beetle and frass volatiles developed during the week after adult eclosion.