The temporal pattern of parasitization of the Angoumois grain moth, Sitotroga cerealella eggs by female Trichogramma evanescens, and the color preference by the parasitoid, were examined in laboratory. Under light-dark conditions, an obvious rhythmic parasitization activity was observed. Parasitization rate was high during the light phase and extremely low during the dark phase. Under continuous light a similar rhythmicity was observed, suggesting that the activity is controlled by an endogenous circadian clock mechanism. In both cases the number of parasitized eggs was maximal on the first day of contact between the parasitoid and the host eggs, and then gradually decreased. Under continuous darkness, however, much fewer eggs were parasitized and no rhythmicity could be discerned. It is thus concluded that although parasitization behavior is probably shaped by an internal oscillator mechanism, light is necessary for the mechanism to be manifested, and some other environmental factors, e.g., darkness can directly modify the behavior, thus reflecting remarkable plasticity. When given host eggs on card pairs of different colors, the female T. evanescens exhibited the following preferences: yellow over green, red or blue; green over red or blue; and blue over red. Given the choice among five different colors, the female T. evanescens exhibited a significant color preference ranking of yellow > green > white and blue > red. The disappearance of such preference in darkness indicates that the parasitoids discriminate between colors. Implications of these findings are discussed.