The digestive tract, malpighian tubes, and reproductive system of the female Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins, are described. The accessory glands of the female reproductive system nearly doubled their length and width after the insects mated. Females producing the sex pheromone, i.e., unmated and feeding (newly emerged as well as re-emerged), possessed longer malpighian tubes than those not producing the pheromone, i.e., unmated and not feeding or mated and feeding. No such differences were found in the similar stages of the males. In arrestment tests to different organs of feeding, unmated females (foregut, midgut, malpighian tubes, and reproductive system), the male test beetles were arrested by the posterior part of the hindgut (rectum) only. When 2 drops of artificial borings from fresh Douglas-fir phloem were added to the organs tested, males were arrested also by the anterior part of the hindgut (small intestine) and by the malpighian tubes. The malpighian tubes of the unmated, feeding females, apparently are associated with the production of the sex pheromone.