The diurnal flight response of the walnut twig beetle, Pityophthorus juglandis Blackman (Coleoptera: Scolytidae), was assessed during two seasonal periods at two sites in northern California. Males and females flew primarily at dusk in response to aggregation pheromone-baited traps during late June/early July, and the percentage of beetles that flew between sunrise and late afternoon was positively correlated with the temperature at 6 AM of that day. Between late August and early November, the diurnal flight pattern was more varied and generally bimodal, though approx. 42 to 55% of the beetles still flew during the dusk period. During either period of the season, there was very little evidence that any flight occurred during the night. Between late August and late September, diurnal flight of both sexes during two-hour intervals was associated with temperature in a Gaussian manner with a threshold at 17–18 °C (62.6–64.4 °F) and peak activity at 23–24 °C (73.4–75.2 °F). The proportion of females in the catches of P. juglandis that responded to pheromone-baited traps ranged from 0.43 to 0.84 but did not vary throughout the day within a seasonal period.