An aqueous mixture of trimethylamine, ammonia, linoleic acid, and indole acidified topH 6 with acetic acid attracts the eye gnatHippelates collusor(Townsend). To study the influence of structural modifications of these active components on attractancy, we bioassayed homologs and analogs of the against field populations of the insects. Primary and secondary aliphattc amines either decreased or did not influence the attractancy of a composition containing ammonia, linoleic acid, and indole acidified with acetic acid topH 6. With the exception of tripropylamine, all tertiary amines tested enhanced the attractancy of the composition. Cyclic amines reduced it.When a low-molecular aliphatic carboxylic acid was used to aCidify a composition containing trimethylamine, ammonia, linoleic acid, and indole topH 6, those acidified with acetic, propionic, orn-butyric acid showed the greatest attractancy.The addition of linoleic or oleic acid to a composition containing trimethylamme, ammonia, and indole acidified topH 6 with acetic acid increased the activity of the composition. Saturated fatty acids usually lowered it. Among unsaturated fatty acids tested, only those having 18 carbon atoms with acisdouble. bond at the 9 positon or 2cisdouble bonds at the 9 and 12 positions were effective in increasing the activity.