Rice in storage in Thailand is often infested with stored insect pests. Local rice milling factories in this region are unwalled employing a traditional open-air ventilation system and therefore insect pests easily attack the stored rice. The factories keep the rice for a few months to a year and do not usually clean the premises after the milling operation. The unclean environment becomes a suitable habitat for reproduction of insect pests, and consequently insect infestations on rice increase during storage. We set up two rooms (cleaned and uncleaned) to examine the effect of cleaning on stored rice grains. Bags of paddy, brown and milled rice were placed in the two kinds of rooms for one year to observe population dynamics of the insects and infestations caused by the pests on the rice. We found fewer insects in the milled rice than in paddy and brown rice but suggest that cleaning of the storage premises may decrease grain losses on all the types of rice.