A sex pheromone was detected in females of Euxoa ochrogaster (Guenee). The method of extracting the pheromone and the olfactometer used for the laboratory bioassay are described. The sex pheromone was not extracted in detectable quantity from 4-day-old unmated females hut was recovered in increasing concentrations from unmated females that were from 7 to 20 days old. The pheromone was recovered also from unmated and Mated females that were collected in the field. Unmated males would not respond to the pheromone until they were 7±2 days old and had been conditioned in the olfactometer for at least 2 days. The male response was greatest between 1.5 and 4 hours after the dark period began; at a temperature of 20±2°C they detected a minimum of 10-3 female equivalents of extract from 17- to 20-day-olcl females. The males survived in the olfactometers for an average of 23 days. Unmated males were stored at 5°C for lip to 36 days after they emerged, with no diminution of their response to the pheromone.