When the dosage of the primary pheromone component (Z)-ll-hexedecenal (Zll-16:AI) of the cranberry girdler,Chrysoteuchia topiaria(Zeller), was varied from 0.03 to 10 mg in field tests, maximum catch was obtained at a dosage of less than 1 mg. When (Z)-9-hexadecenal (Z9-16:AI)was present at 0.3, 1, or 3% of Zll-16:AI, trap catch was proportional to the logarithm of the dosage over the same concentration range. The amount of Z9-16:Althat produced maximum trap catches within calculated confidence limits of each dosage ranged from 0.2 to 2.1% when the dosages of Zll-16:Al were 0.3, 1,3, and 10 mg. Z9-16:AI at 1% always was included within the confidence limits of these dosages. The two higher dosages captured significantly more males than the two lower dosages, but trap catches were not significantly different between dosages of 0.3–1 and 3–10 mg. The reduced trap catch by Zll-16:AI alone for dosages above 1 mg was the result of the absence of an important pheromone component. In flight tunnel studies, Zll-16:AI induced limited plume-oriented flight (19% of tested males) at a dosage of 0.03 mg only, and males did not reach the source. The percentage of males exhibiting upwind flight in the plume increased dramatically when Z9-16:AI was present. At a dosage of 0.3 mg, upwind flight in the plume was 0% for the single component and 90% for the two components. Maximum upwind flight occurred at dosages from 0.03 to 3 mg and, as occurred in field tests, the Z9-16:AI at 1% of Zll-16:AI always was included within the calculated confidence limits for these dosages. The 3-mg dosage of Zll-16:AI reduced the number of males landing on the septa.