The phototactic response of Trogoderma granarium Everts was studied in a specially designed visiometer. In both single-sex and mixed-sex groups, the beetles showed a highly significant (P<0·001) photonegative response. A higher proportion of females than males were found on the dark side in single-sex and mixed-sex groups. The influence of the group size on the phototactic response in this species was evaluated using special dark-light choice chambers. The photonegative response was found to be significant in groups of 15-40 male beetles and 5-40 females. Smaller groups did not show any significant response to light. Aggregation occurred in this experiment especially when large numbers of beetles were used. The photonegative response was tested at different temperatures in dark-light choice chambers at constant relative humidity. In both sexes, the intensity of the photonegative response decreased with increase in temperature. The relationship between the phototactic and the hygrotactic responses of T. granarium was studied in appropriately modified dark-light choice chambers. The beetles reacted positively to the drier side of the relative humidity gradient (20-80 per cent r.h.) when tested under total illumination or in darkness. However, illumination of only the drier side of the relative humidity gradient caused a photonegative response and distinct reversal of the hygroreaction.