Abstract: Date palm fruit stalk borer, Oryctes elegans Prell. (Coleoptera: Scarabaeidae, Dynastinae), is one of the most serious pests of date palm in the Near- and Middle East and causes economic losses in Iran and Iraq. Adults cause damage by mining the stalks of fruit bunches. In this study, sexual dimorphism and nocturnal activity patterns of O. elegans were investigated under semi-field conditions. Results showed that the best criterion to differentiate males and females is the lateral parts of the pygidium, which is angled and opaque in the females and smooth, round and shiny in the males. Observations inside the artificial galleries of the offshoot showed that the beetles were active from one hour after sunset until sunrise. Maximum activity in both sexes was between 22:30 and 1:30.